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A design guide for vacuum drainage systems, for use in conjunction with EN 72709: Vacuum drainage systems inside buildings

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Contents

1 1.1 1.2 1.3 1.4

2 2.1 2.2 2.3

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11

4 4.1 4.2 4.3 4.4

INTRODUCTION ...................................................................................................................... 3 Scope .......................................................................................................................................... 3 Background ................................................................................................................................ 3 Vacuum drainage systems compared to gravity drainage .......................................................... 3 Applications ................................................................................................................................ 3

SYSTEM DESCRIPTIONS .................................................................................................... 4 Definitions of terminology ........................................................................................................ 4 The vacuum transport process .................................................................................................... 5 Operation of system .................................................................................................................... 5

BASIC COMPONENTS .......................................................................................................... 6 Interface units ............................................................................................................................ 6 Vacuum toilets ............................................................................................................................ 8 Collection tanks and vacuum generation .................................................................................. 8 Ejector unit ................................................................................................................................ 8 Vacuum station .......................................................................................................................... 9

Controls .................................................................................................................................... 10 Column tank ............................................................................................................................ 10

Check and isolation valves ...................................................................................................... 11 Pipework .................................................................................................................................... 12

Vacuum generating and forwarding pumps ............................................................................... 9

Combined vacuum generator and forwarding pump .............................................................. 11

DESIGN .................................................................................................................................. 12 Design requirements ................................................................................................................ 12 Design criteria ................................... : ...................................................................................... 12 Maintainability ........................................................................................................................ 15 Noise control ............................................................................................................................ 15

BIBLIOGRAPHY .................................................................................................................................... 16

DESIGN EXAMPLES

In the course of producing this guidance. the following sites in the UK have been studied:

m The Earth Centre. Doncaster

m Lords Cricket Ground. London

m McAlpine Stadium. Huddersfield

m Sainsbury Supermarket. Sevenoaks

m Waitrose Supermarket. Gloucester Road. London .

0 Institute of Plumbing 2001 Published by Institute of Plumbing . Produced by BRE

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Parameter

Pipe size (mm)

! . . , ' . ' I

Conventional

Branches 32-100 Stacks 100-150

(gravity)

1 INTRODUCTION

Branches 32-100 Stacks 100-150

To a fall

Regular planned servicing of pumps and interface units

1.1 Scope The aim of this publication is to act as a guidance document for vacuum drainage systems, for use in conjunction with EN1 2109: Vacuum drainage systems inside buildings. This guide is aimed at designers, installers and building services students and lecturers who may be considering using a vacuum drainage system or who wish to know more about them.

This document covers vacuum drainage systems within buildings, and does not cover their use outside buildings, or vacuum sewerage. The principles of vacuum drainage are covered, however the design details, such as pipe sizing, are not dealt with as such matters would be designed by a vacuum drainage system specialist.

The terms used in this document are as defined in EN12109. However, a few of the definitions from the European Standard are included in section 2 to aid the reader's understanding.

Discharge from valves 32-50 Service connection 38-90

Flexible arrangements, with minimal gradients or sawtooth profile. Vertical upward flow sections 'lifts' can be used

Regular, planned servicing of pumps and interface units

1.2 Background Generally, the modern vacuum drainage system is believed to have been started by the development of the vacuum toilet of Mr Joel Liljendhal in the mid 1950s. The concept of his system was to separate the heavily polluted waste and the lightly polluted water (black and grey water) for separate treatment. The vacuum toilets used only about 1 litre of water per flush, improving the efficiency of the system and making treatment easier. This two-pipe system, although environmentally friendly, met with quite a lot of resistance because houses needed to be re-plumbed and the vacuum toilet required more maintenance than a conventional toilet. This lead to the development of the single-pipe vacuum drainage system generally employed today.

At time of installation and throughout lifetime of building

May require additional Pumps

7.5 litre flush WC

1.3 Vacuum drainage compared to gravity drainage Table 1 summarises the differences between conventional drainage systems and a vacuum drainage system.

At time of installation and throughout lifetime of building

Flexible layout makes installation simple

N/A

1.4 Applications Particular consideration should be given to the use of vacuum drainage in the following circumstances: 0 water shortage or other reasons for reducing

water consumption 0 limited sewerage capacity 0 where separation of black and grey water is

desired, e.g. where grey water is reused 0 where separation of wastewaters is desired,

e.g. for different treatments

Conventional water consumption WCs

Low water consumption WCs

Loading of sewerage system

7.5 litre flush WC

6 litre flush WCs

Dependant upon appliances installed

Pipeline gradient

~~~~

Dependant on pumping rate

To a fall

Discharge from forwarding pumps can be times to coincide with low flow periods

Maintenance requirements

Energy requirements

Whilst the original systems were developed for domestic sewerage transportation, the systems evolved in two different areas; transportation and land-based systems. The majority of work has been in the transportation sector, and the marine industry continues to be the major user of the system, where the need to conserve wholesome water, and the problems of confined pipework runs and sewage disposal are paramount. For the same reasons, this technology was adopted by the other forms of mass transport, i.e. airlines and railways. Today some SO00 ships, from yachts and cargo vessels to ocean going cruise liners, SO major airline companies, and over 1000 train installations use vacuum drainage systems.

The building sector has been slow to adopt this new technology, but with the restrictions placed on new projects such as small conduit, ceiling voids and service ducts, and the growing awareness of the need to limit water consumption, the system is finding a place in the building sector.

Negligible, only after

At time of installation

Retrofit or extension of system within building

May be difficult to accommodate pipework and falls

~~

I Vacuum Conventional (DumDed)

6 litre flush WCs 1.5 to 3 litre flush vacuum toilets

Table 1 - differences between different types of drainage systems

(3

1 Advantages I Disadvantages Low installation costs

Environmentally safe

Electrical power only required at vaciiiim station

Always self cleansing

,

High component costs

Mechanical components - possibly for failure

Skilled design, installation and maintenance required

Regular maintenance required

No possibility of vermin in pipelines Possible water saving technique if vaciiiim toilets are used

Standby facilities required

Require area for situation of vacuum tanks and vaciiiim generation equipment

I High water velocities prevent deposits in pipework I High velocity water may cause transient plumbing noise I Minimise risk of leakage

Can use small diameter lightweight pipes that can be installed without a continuous fall

I Vertical lifts are possible I I I Ability to easily separate grey and black water I I

High turn around time - no need for cistern to refill for subsequent flushes I

Table 2 - Advantages arid disadvantages of vacirirni drainage systenis

0 in hospitals, hotels, office buildings or other areas where congested usage occurs when flexibility in pipe routing is required to drain appliances or where frequent pipe layout changes are expected

0 building refurbishment

0 where drainage by gravity becomes impractical

0 in complex building structures, and

0 in penal installations where isolation and control of the appliances is necessary to prevent concealment of weapons and drugs.

The advantages and disadvantages of vacuum drainage systems are contrasted in Table 2.

1.4.1 Black and grey water The collection arrangements and small bore pipework of vacuum drainage systems provide the possibility of easily separating grey and black water. This was one of the original aims of the invention by Mr Liljendhal. This would be of particular advantage i f sewerage capacity was at a premium as the grey water could be run to a water course after appropriate treatment. Also, it would be of advantage if there was a requirement to use the grey water for reuse or irrigation.

1.4.2 Retrofit and new build flexibility (See Fipre I ) When conventional gravity drainage systems are extended as in refurbishment work, the existing gravity drainage system can be fed into the vacuum drainage system. This may be achieved by the use of a sump into which the wastewater from the gravity system drains. When sufficient water has accumulated in the sump, an interface valve will open allowing the wastewater to enter the vacuum drainage system. This arrangement, as shown in Figure 1, can also be used to collect rainwater or as an interface between a building with conventional drainage and a vacuum sewer.

Level sensor Pipe

Controller

lncomina gravity- \ drain

I I Collection Chamber

Emptying

Figure 1 - Siriirp receiving wastewater fioin gravity systeiiis

2 SYSTEM DESCRIPTIONS An analogy of a vacuum drainage, or sewerage, system is shown in Figure 2 as an inverted water supply system in which the water flows backwards. The usual operating vacuum range of a vacuum drainage system is -0.5 to -0.7 bar gauge.

2.1 Definitions of terminology A selection of the fundamental definitions that are required to understand the terminology of vacuum drainage systems are listed below. A complete set of definitions are contained within the European Standard EN 12 109.

SUPPLY TANK WATER OUTLET VALVES

1 7

Pressure plpes Tank Pump Valves I

I VACUUM INTERFACE VALVES COLLECTION TANK

Figure 2 -Analogy of a vacuum draitiage systet?i

Buffer volume The storage volume of the interface unit which balances the incoming flow of wastewater to the output capacity of the discharge valve.

Controller

Interface valve

Lift

The device which, when activated by its level sensor, opens the interface valve and, after the passage of wastewater and normally air, closes the valve.

A valve which admits the flow of wastewater only, or wastewater and air, into the vacuum drainage system pipeline.

Section of vacuum pipeline with an increase in invert level in the direction of flow.

Reforming pocket A low point in the piping profile installed intentionally to produce a controlled slug flow.

Service connection The section of vacuum pipeline connecting an individual interface unit to the vacuum main.

Slug

Vacuum

Vacuum station

An isolated quantity of wastewater flowing full bore through the vacuum pipeline.

Any pressure below atmospheric.

An installation comprising vacuum generator(s), a means of discharge and control equipment, that may also incorporate vacuum vessel/holding tank(s).

2.2 The vacuum transport process An understanding of the vacuum transport process is helpful to the system designer. As long as no interface unit is operating, little wastewater transport takes place. All wastewater remaining in the vacuum pipework will drain, by gravity, into the reforming pockets when all upstream interface valves are closed. When an interface valve opens, the differential pressure between the vacuum in the system and atmosphere forces the wastewater into the vacuum pipework. Whilst accelerating, the wastewater is transformed into foam and soon occupies only part of the vacuum pipe cross section so that the momentum transfer from air to water takes place largely through the action of shear stresses. The magnitude of the propulsive forces starts to decline noticeably when the interface valve closes, but remains important as the

admitted air continues to expand. Eventually, friction and gravity bring to rest the wastewater at the low points of the pipework system; such as reforming pockets and at the bottom of pipeline lifts.

The vacuum drainage system transports wastewater by means of atmospheric pressure acting against vacuum. The operation of the system is described below.

2.3 Operation of system When an interface valve is opened, the difference in pressure between the main pipeline and the interface unit drives the water out of the buffer volume and into the pipeline. A quantity of air, several times the volume of water, follows the water into the pipework before the valve closes. This creates a large local pressure differential that accelerates the water vigorously in the service connection. This movement of water takes place in the direction of lower pressure, so initially the flow is both upstream and downstream, but the connection angle and gradient have directional effects on the water flow. Within any low points, or pockets, in the pipework slugs of water will form.

After a number of repeated accelerations of slugs of water, the air has lost most of its kinetic energy and cannot create any more pumping action. This is called the Reach of the Interface unit. The reach depends upon many things, e.g. buffer volume, valve timing, aidwater ratio, pipe diameter and length. For many systems, the transportation distance is normally within the reach of every interface unit. However, for longer systems (i.e. greater than lOOm pipe length), and for further transport, several interface units may be required to operate in sequence to supply the necessary volume of air. At low flow conditions, for example in the middle of the night, sequential operation might not occur. Some vertical lifts may then become full, or almost full, of water. To restore the required dynamic flow regime necessary for vacuum transport, one or more adjacent interface units have to operate to supply the air, or a separate valve unit that will supply air on demand only must be installed.

When designing systems greater than lOOm in length (from the valve to the vacuum station), a series of reforming pockets must be used. These minimise the break-up of the wastewater slug and reform that portion of the slug that remains in the piping between interface valve discharges. The reformed slug is then propelled by the air admitted during the next discharge.

Once the interface valves have operated, the discharge travels to the vacuum station, normally located at ground or basement level. Air is discharged to atmosphere only from the vacuum station. From the vacuum station, the wastewater is pumped automatically to the building outfall connection, to discharge into the external drainage system by gravity.

To design a reliable and economic vacuum drainage system, it is necessary to generate sequentially high acceleration and self-cleansing velocities with the least amount of energy.

NOTE: A vacuum drainage system is NOT a reversed pressure system where all the water would be accelerated simultaneously.

3 BASIC COMPONENTS The vacuum drainage standard considers the Vacuum Drainage System in four system elements: 0 the automatic interface units (AIU) 0 the vacuum toilets 0 the pipework, and 0 the vacuum station.

3.1 Interface units

3.1.1 Description The valves that form the interface between the vacuum drainage lines and the appliances can be used directly with some appliances and with buffer volumes for others. When used with buffer volumes, level sensors and controllers the valves are termed interface units. Most interface units operate automatically and are known as automatic interface units (AIUs). Although AlUs are operated by air, non-automatic units may use electricity to control their operation.

There are various sizes of interface valve up to about 100mm bore. The larger valves are used in vacuum sewerage systems.

Typically a complete interface unit is composed of a buffer volume of varying size, a sensor to sense the wastewater level in the buffer volume, a controller which operates a pilot valve to open and close a vacuum supply line to the interface valve. In many designs of interface unit, level sensors and controllers are combined into one device.

3.1.2 Operation Normally, interface valves would be operated by vacuum but may be operated by electricity if appropriate. Although the operation of all automatic interface valves is similar, vacuum toilets operate differently (see section 3.2). A typical sequence of operation for an automatic unit, with an air tube level sensor and combined controller/sensor unit, is: 0 liquid enters the holding tank by gravity, as the

liquid level rises in the sump it pressurises air in the sensor pipe

0 this air pressure is transmitted through a tube to the controller/sensor unit

0 the air pressure operates the controller/sensor unit, which operates a pilot valve to open the vacuum supply line to the interface valve's operating mechanism

3.1.3

3.1.3.1

0 this opens the interface valve and activates a

0 after the set time period has expired, the buffer timer in the controller

volume will have been drained and sufficient air to transport the wastewater will have been admitted

0 the controller/sensor unit operates the pilot valve to close the vacuum supply line to the interface valve and to admit air to the valve operating mechanism

0 the interface valve closes.

The arrangement of the components may vary according to the design of the installation. In general terms, the buffer volume gets larger as the inflow rate increases. The buffer volume can be a separate tank of various materials suitable for the application. It may also be possible to use part of the gravity drainage system that feeds the interface valve as a buffer volume.

There are several types of interface valves suitable for vacuum drainage, e.g. piston valves, pinch valves and diaphragm valves. All interface valves are normally closed and require vacuum to be applied to open the valves.

Pinch (See Figure 3) The pinch valve comprises a flexible tube running between the inlet and outlet of the valve body, a spring-loaded closing mechanism that requires compression to release the tube, an airtight body with flexible sleeved ends that are attached to the ends of the closing mechanism.

Rubber Sleeve

~ ' Rubber Sleeve'

INTERFACE VALVE

Vacuum Side

Figure 3 - Pirich valve

Operation The valve is opened when the controller applies vacuum to the inside of the valve body. The external air pressure then compresses the closing mechanism which allows the tube to return to its natural open state and connect the appliance to the vacuum system. Air at atmospheric pressure then forces the wastewater through the open pinch valve and into the piping.

When the controller disconnects the vacuum from the inside of the body, it allows air to enter the body which assists the spring to return the ends of the mechanism to their rest positions and pinch the flexible tube closed.

3.1.4 Spoon (See Figure 4) The spoon valve comprises a ridged tube with a movable, spoon shaped obturator that is held closed by a spring-loaded piston. The spoon’s back is against the appliance side of the valve and its tip locates in a depression in the tube. Around the piston is an airtight chamber that contains servo valves which will admit air or vacuum to the piston.

Spring Valve controller

_ _ _ _ _ _ - - - - - - - - -

Piston

1 Figure 4 - Spoon valve on the outlet of a vacuum toilet

3.1.4.1 Operation When the controller applies vacuum to the piston’s servo, a valve will open in the chamber and the applied vacuum will cause the spoon valve to open, compressing the spring. Air at atmospheric pressure then forces the wastewater through the open spoon valve and into the piping.

When the controller ceases to apply vacuum to the piston servo, another valve opens in the chamber to admit atmospheric pressure air into the chamber. This allows the spring to return the spoon to its closed position.

3.1.5 Piston (See Figure 5) The valve consists of a Y fitting, with a straight path between the inlet and outlet. At the end of the angled arm are two chambers separated by a diaphragm connected to the sealing piston. In the upper chamber is a spring. The piston is angled so that it will not obstruct the pipe when it is raised and also so the seating will not cause a constriction in the pipe.

Q

4. Suction pipe 5. sensorpipe 5 Depth signal 7. Wfstewater inlet

1. Controller 2. P a l under vacuum 3. Pal at atmospheric wwsure m - Wastewater w

Air 0 Q Vacuum I

3.1 S . 1 Operation In the closed position, the two chambers are maintained at atmospheric pressure by a controller (1). The vacuum in the pipelines and the spring in the upper chamber hold the piston on its seat. When the controller receives a signal (6) from the sensor pipe (5) due to a high level of wastewater in the pump, applies vacuum to the upper chamber, the diaphragm is forced into the top chamber, pulling the piston and opening the valve. Air at atmospheric pressure then forces the wastewater, up the suction pipe (4), through the open piston valve and into the piping.

The controller has an adjustable timer that controls the valve’s rate of closing by slowly admitting air to the upper chamber until the spring returns the piston to the closed position.

3.1.6 Modified diaphragm (See Figure 6) A diaphragm valve uses a shaped flexible diaphragm to seal against an angled seating. The means of retracting the diaphragm varies between designs. The modified diaphragm valve uses a spring for rolling the diaphragm onto the seating while a conventional diaphragm valve may use a less simple rod mechanism.

Figure 6 - Diaphragm valve

The modified diaphragm valve comprises a straight rigid tube with a diaphragm attached to one side, over which is a chamber that supports a closing spring and can be connected to vacuum. One end of the spring is attached to the diaphragm’s sealing face and the other is located onto the chamber so that the naturally straight spring is held in a curved state.

3.1.6.1 Operation In the closed position, the diaphragm is held against the vacuum seating by the spring and atmospheric pressure. When the controller applies vacuum to the chamber over the diaphragm, the diaphragm seating face retracts into the top chamber. This creates a clear passage opening the valve. Air at atmospheric pressure then forces the wastewater through the open diaphragm valve and into the piping.

The controller has an adjustable timer that controls Figure 5 - Automatic interface unit with piston valve

the valve’s rate of closing by slowly admitting air to the chamber. This allows the spring to return the diaphragm to its closed position.

3.2 Vacuum toilets A vacuum toilet uses air instead of water to remove the contents of the bowl, and is a form of interface valve. Usually, it includes a rinse rim and the toilet’s controller may have a memory function so that it will operate as soon as there is sufficient vacuum available. A typical vacuum toilet is shown diagramatically in Figure 7.

3.2.1

air inlsf liner

nnse

vacuum hose - vacuum pipe

ring

\ / loilef bowl 1177

Figure 7 - Vacuum toilet controls and operation

Operation of dry vacuum toilet Where minimal water use is required, a dry vacuum toilet may be used. The water consumption of this type of toilet is typically 0.2 to 0.4 litre per flush. The dry vacuum toilet retains no water in its bowl, as the flush valve closes before the discharge valve closes. The cleansing of the bowl is usually inferior to that of a wet vacuum toilet.

3.2.2 Operation of wet vacuum toilet Typically, when operating a flush button, pressurised water is introduced into the bowl through a water valve and a rinse ring with holes to clean the bowl. Simultaneously the discharge valve opens and the pressure differential in the piping forces the contents through the valve. Before the discharge valve closes, air is drawn into the pipe. The flush water valve stays open for about two seconds to re-establish the water pool in the bowl. The typical water consumption for this timing sequence would be 0.8-1.5 litre per flush. Vacuum toilets may be re- flushed in less than a quarter of the time taken for a conventional WC to refill, on average, a vacuum toilet will take 3 seconds to complete a flush cycle.

3.3 Collection tanks and vacuum generation These have three functions: 0 generate vacuum a receive and forward the wastewater, and 0 control and nionitor the system.

3.3.1 Station sump valve The plant room containing the vacuum generator may be provided with a sump to collect washdown water. The sump is automatically

emptied by a 50mm or 75mm valve into the vacuum collection tank. An isolation valve may be fitted between the vacuum tank and the interface valve.

3.3.2 Standby generator Unlike most gravity drainage systems, vacuum drainage systems require electrical power for operation. Hence, there may be a requirement to provide an alternative power supply to operate the system in case of a primary power supply failure.

A vacuum drainage system should require only one source of power located at the vacuum generation plant room. The standby generator is usually rated to provide 100%) standby power for discharge pumping and vacuum generation. The generator should be arranged for automatic start upon failure of the main power supply.

3.4 Ejector unit (See Figures 8 and 9) Ejector units are used on small systems, which require approximately 40m3 of air an hour at peak flow. They have the advantage of having a lower capital cost, being small in physical size and with fewer working parts than vacuum pumps, and are easy to maintain and operate. However, they are less power efficient than a vacuum station and, therefore, are more expensive to run. The control of this kind of vacuum station is similar to a conventional pumping station. These units can also receive discharges from gravity drainage systems directly into the tank. However, wastewater containing high levels of detergents may cause foaming problems.

Operation (The ntiinhers refer to Figure 9) The ejector unit creates a vacuum in the pipework by means of an ejector (2) and pump (3). Wastewater is pumped from the tank (1) through the nozzle of the ejector creating a venturi effect, whereby air and waste are inducted from the

3.4.1

, , - Vacuum drainage pipeline

Check Valve

B

Figure 8 - Schematic of ejector

1. Atmospheric lank 2. EVAC ejector 7. Discharge valve, automatic 11. S h u t 4 valve 3. EVAC discharge pump or manual 12. Shut-oH valve 4. Pressure switch 8. High level switch 13. Control equipment 5 Vacuum gauge 9. Sewage -discharge line

6. Vacuum drainage pipeline 10. Low level switch

Figure 9 - Ejector unit

vacuum drainage pipework, mixed with the wastewater jet and passed to the collecting tank. The induction of air from the drainage pipework creates the vacuum within the pipework. When the level of water in the tank reaches the high level switch (8), the collected wastewater is transferred to the sewer by using the same pump(s), and by diverting the flow through a discharge valve (7). When the water level in the tank has dropped to the level of the low-level switch, the discharge valve will close. When the set vacuum has been developed in the pipework, the vacuum switch (4) will cause the pump(s)(3) to stop. The vacuum will be maintained in the pipework by the check valve at the end of the vacuum drainage pipeline, within the ejector. As the tank is vented to atmosphere, wastewater from a conventional gravity system can be introduced into the tank and then pumped to the sewer during the normal discharge cycle.

3.5 Vacuum station (See Figures 10 and 1 1 )

Figure 10 - Vacuum station

Although vacuum stations may be used with simjlar systems as the ejector unit, they are used mainly for larger systems, i.e. greater than 40mR of air. They are large units with a higher capital cost but typically are cheaper to run than ejectors.

The machinery installed is similar to that of a conventional wastewater pumping station or lift station, and consists of a collection tank, wastewater forwarding pumps, vacuum pumps, controls and

I VACUUM &.TEM 3 COLLECTON SYSTEM z FORWAMNG SYSTEM

alarms, and where required a standby generator.

Vacuum stations may be built from a package of equipment provided by the manufacturer of the system. Package vacuum stations may be pre- fabricated on a skid at the manufacturer's plant and delivered to site fully assembled and tested.

The vacuum receiver tank size and/or number of tanks depends on the number of appliances connected to the system and the excepted frequency of discharge. Each tank incorporates level indicator switches that control the discharge pumps automatically, vacuum regulator switches which control the vacuum pumps and level alarms which can be audible or connected to the building management system.

3.6 Vacuum generating and forwarding pumps

3.6.1 Vacuum generating pumps Vacuum pumps of the liquid ring and sliding vane types are both suitable for use in vacuum drainage systems. Vane-type vacuum pumps are recommended for most projects, as they are more efficient, i.e. they have a greater throughput of air and are less temperature sensitive than similarly powerful liquid ring pumps.

The maximum vacuum provided by a liquid ring pump often will not exceed -0.8 bar gauge, whilst the maximum vacuum of a vane pump will typically be closer to -1 bar gauge. This will affect the choice of pumps where vacuum levels of a greater magnitude than the normal -0.5 to -0.7 bar gauge operating range will be required, or for projects at high elevations where atmospheric pressure is lower.

A vacuum switch attached to the pipework and adjustable timer are used to control the vacuum pumps. A second vacuum switch may control a low vacuum alarm signal. These switches are fitted with stainless steel bellows to protect against corrosion from any gases evolving from the wastewater.

3.6.2 Forwarding pumps (See Figure 12) Forwarding pumps are required to discharge the collected wastewater to the external gravity sewerage

Figure 11 - Vacurrm station Figure 12 - Balance line for forwarding pump

system. These pumps are designed to operate with a large pressure differential across them with their inlets under vacuum. The size of the forwarding pump is a function of the following: design peak flow, volume to be discharged, and the permissible discharge rate for the receiving sewer.

To enable some forwarding pumps to work, a vacuum balance line may be required downstream o f the discharge pump to reduce the pressure difference across the pump (a balance line is not required with an ejector systems). To prevent loss of vacuum when the pump is not discharging, a check valve is required in the discharge pipework downstream of the connection of the balance line see Figures 11 and 12.

3.7 Controls

3.7.1 System controls I

The vacuum drainage system control panel contains the main power switch and the pump operating system, which includes magnetic starters, overload protection, control circuitry and hours run meters for each vacuum and forwarding pump. A data recorder may be built into this panel, as well as the collection tank level control relays.

Alarm and telemetry systems may be included if also required.

3.7.2 Pump controls I The controls should be designed so that, where

standby pumps and collection tanks are installed, both the vacuum and forwarding pumps alternate their use and are interconnected and controlled to allow them to be used with either tank automatically.

The pump controls will include logic controllers that will be connected to the various level and vacuum sensors. The signal to start the discharge comes from the high-level switch in the collection tank, the stop function is either controlled by a low- level switch or timer. For example, when the high- level sensor indicates to the logic control that water has risen to the high level, this sequence of operation will commence:

0 the controller will close automatic valve in balance line, where fitted, and start the forwarding pump

0 if level does not fall within the pre-set time, a second pump will be started or an alarm generated

0 if level reaches the high level alarm sensor, then an alarm is given and the vacuum system is shut down

0 when the water level has fallen to a low-level sensor or after a pre-set time period, the controller will stop the forwarding pump(s) and open the automatic balance line valve, if fitted.

In systems where only black water is being collected, it is prudent to use the second forwarding pump as a circulation pump. This circulates sewage within the collection tank and breaks up any solids which may have formed on the surface of the wastewater. This operation should be programmed into the logic system as the first step in the discharge cycle.

3.7.3 Collection tank level controls Level detectors are available in various forms, some are float switches, others are fixed probes that may be conductive, inductive or capacitative. Where a lot of condensate is being collected, for example in supermarkets with chiller cabinets, the mineral content of the water may affect the operation of the system and conduction probes may need to be used.

Level detectors, of some form, are fitted to all collection tanks. The signals from the six common detectors control the discharge pumps and alarms as follows, in ascending order of height from the base of the tank: 1. earthing probe, or sensor float switch or similar 2. both forwarding pumps stop 3. lead forwarding pump start 4. assist forwarding pump start 5. high-level alarm 6. high-level cut-off - stop vacuum pumps.

3.7.4 Vacuum gauges It is important that all vacuum gauges be specified to indicate gauge pressure and have stainless steel bourdon tube and socket.

Vacuum gauges should be provided at the following locations: 0 the side of the vacuum moisture removal

tank (where fitted) 0 the collection tank, and 0 one gauge on each incoming vacuum main

or header.

It is important that these gauges are located above the incoming pipes and in a position that is easily viewed

wc WC

Separator

Water column

Trap sump wlumn

To sewer

Column Tank

Figure 13 - Column tank installation

from the operating position of the isolation valves. Column tank (See Figure 13) Wastewater may be forwarded by other means than forwarding pumps, e.g. by gravity i f a sufficiently high water column can be arranged to prevent backflow of air into the vessel. This is the principal employed in the column tank. For example, if the system is operating at -0.5 bar, the height of the column needed to balance the vacuum would be 5m. The volume of the sump of the trap would need to be 5 x 7Lrz m3, where r is the internal radius of the column pipe.

At the top of the column is an air water separator, and at the bottom a water trap sump of sufficient capacity to balance the volume of the water column created by the negative pressure (vacuum). The wastewater is forwarded to the gravity sewer continuously. This system is useful particularly when the negative pressure is below 0.6 bar. Column tanks of about 2m may be used in apartment buildings when using small vacuum generators such as a combined vacuum generator and forwarding pump and discharging into gravity drainage stacks.

3.8

3.8.1 Vacuum reservoir/moisture removal It is important to prevent moisture entering the vacuum generating pumps to protect their mechanisms and prolong their service life. This is particularly a problem with vane-type vacuum pumps which often have a low tolerance of any moisture carried over into them. Moisture carryover from the collection tank can be prevented by using a moisture removal tank or an automatic condensate trap.

I 3.9 Combined vacuum generator and forwarding pump (See Figure 14)

,

Figure 14 - Diagram of comblnedgenerator and forwarding pump installation

A combined vacuum generator and forwarding pump or ‘vacuumarator’ is a screw vacuum pump with liquid ring seal with a macerator for breaking-up any solids passing through it. The macerator consists of one rotating knife fixed to the shaft and one stationary knife fixed to the suction chamber.

The combined vacuum generator and forwarding pump: U generates vacuum 0 macerates solids, and 0 pumps wastewater in the same operation.

A combined vacuum generator and forwarding pump can generate vacuum directly on the pipeline to an appliance and discharge to a gravity system in the same operation. Vacuum tanks or collecting tanks are not required normally. Combined vacuum generators and forwarding pumps can be used for all size of systems. The size and number of combined vacuum generator and forwarding pumps to be used depends upon the required capacity. Combined vacuum generators and forwarding pumps have a small footprint compared to conventional vacuum stations and can be located in small ducts. A combined vacuum generator and forwarding pump is more power efficient than an ejector system, but a large number of combined vacuum generator and forwarding pumps would be more expensive to purchase and run than a comparable vacuum- station-based system.

3.9.1 Operation When the combined vacuum generator and forwarding pump is filled with liquid and started, a liquid ring is created around the rotor. The depth of the liquid ring is governed by the size of the opening in the end plate on the pressure side. This opening is arranged such that the created liquid ring touches the rotor hub on one side and the rotor tips on the other. This arrangement creates a series of progressive crescent-shaped cavities travelling from the vacuum to the pressure side. Air and wastewater are drawn into these cavities and transported through the vacuumarator. Any solids within the wastewater are macerated by the integral macerator before it enters the pump body.

3.10 Check and isolation valves A check valve is installed in each vacuum pump suction line to maintain the vacuum in the system. Check valves are also fitted on the discharge from a vacuum discharge pump, and often are fitted on the service connection from an appliance.

Isolation valves are fitted to all forwarding and vacuum‘ pumps to allow their removal without disrupting the system. Also, they are fitted in strategic locations to enable sections of a system to be isolated for service. Isolation valves should be suitable for vacuum use and may be of the eccentric plug type or resilient face gate type and have a clear opening of not less than the nominal diameter of the pipe. Both check and isolation valves must be capable of withstanding 0.8 bar gauge vacuum, when open, and a differential pressure of 0.8 bar, when closed on a functioning system.

3.11 Pipework Usually, stainless steel and thermoplastics (ABS, HDPE, PVCU or MDPE) pipes are utilised for the construction of the vacuum pipelines - the selection of pipeline material is dependent upon its location and the characteristics of the wastewater. All pipes used should be suitable for vacuum, and the minimum pressure rating for thermoplastics should be 10 bar, but higher ratings shall be used i f the pipe has an initial ovality; if progressive deformation or long-term loss of strength due to high temperature is likely to occur. The velocities of water within the pipework and the resulting percussive effects at changes of direction lead to the requirement for such pressure rated pipe.

Standard manufactured fittings are used where available; Y junctions for incoming branches should be 45" and reducers be concentric.

Generally, joints should be smooth and protrusion free to ensure full bore flow conditions.

Not all rubber ring pipe joints are suitable for vacuum systems. The manufacturer should supply a guarantee along with the test certification that the products are appropriate for vacuum drainage applications.

4 DESIGN

4.1 Design requirements The system should be designed to accept discharges from all appliances planned to be connected to the system. The designer should take into consideration any known possible future additions or modifications to the system to avoid future installation and operating constraints.

There are seven factors that must be considered by the designer and the equipment supplier when designing a vacuum drainage system. They are: 0 health and safety 0 availability 0 reliability 0 maintainability 0 noise and odour control 0 energy economy, and 0 fire resistance.

4.1.1 Health and safety The design must be such as to reduce hazards to an acceptable level and be subject to a risk assessment. This may be achieved by the use of safety devices, warning devices or, if no other option is available, by the use of special procedures such as always having two operatives to check an operation.

4.1.2 Availability Availability is the ratio of operating hours to operating hours plus hours out of service. This measure can be applied to the whole system, part

of the system or just a component. The aim is to achieve high availability by having high reliability, redundancy for key components, i.e. duplicate back-up pumps and tanks, and a system that is simple and straightforward to maintain. In practice, isolation valves may be fitted to each interface unit. Interface valve cycle times are normally limited to less than about 10 seconds so that the system's vacuum recovery time is minimised.

4.1.3 Reliability Reliability is dependent upon the number of valve cycles per day and the quality of the interface valves and the vacuum station equipment. For example, an automatic interface unit would be expected to have a reliability of 250,000 mean cycles between failures when installed in a correctly maintained system.

4.1.4 Maintainability Maintainability is an important factor in the design and calculation of the overall running costs. It includes the provision of access to interface valves, units and any tank or pipework cleaning eyes, the maintenance schedules an.d estimated repair times. (See section 4.3 for more details.)

4.1.5 Noise and odour control Noise from the system is dependant upon the choice of pipe material, the method of fixing, the air to water ratio and the operating vacuum. Secure pipework brackets with rubber inserts will reduce noise. The air discharge from the system should be sited to discharge externally (in accordance with National Building Regulations).

4.1.6 Energy economy Vacuum drainage systems consume little energy, but by careful design and commissioning the consumption can be minimised. To minimise energy consumption, the design should avoid high lifts and high air to water ratios, use high efficiency components, and use a control system which will detect air leaks.

4.1.7 Fire resistance Any system that includes pipes and frequent penetration of walls could contribute to the spread of fire. By the appropriate selection of materials and, where needed, firestops, the design can made resistant to fire. However, as an operating vacuum system is maintained below atmospheric pressure, it would not contribute to the spread of fire in the same manner as a conventionally ventilated gravity system.

4.2 Design criteria In order to design a vacuum drainage system, the following basic parameters should be determined and obtained: 0 service life expectancy 0 type of building

0 number of people the system is to serve 0 types, number and location of appliances to be

0 wastewater temperature range (high connected

temperature grey water discharges shall be specified concerning temperature, flow, batch volume and frequency)

0 ambient temperature range within which the system shall operate

[7 minimum vacuum level required to operate the interface units and vacuum toilets

0 air to water ratios required for the interface units 0 air consumption of vacuum toilets, and 0 permissible leakage factors.

The following parameters are required to calculate the pipe sizes and system layout. They should be determined by the designer and equipment supplier for each system: 0 total wastewater flow 0 vacuum toilet flush frequency [7 dynamic losses between the vacuum station

and the furthest appliance on each pipeline 0 static losses between the vacuum station and

the furthest appliance on each pipeline 0 required operational value of vacuum 0 required vacuum generator capacity 0 required forwarding pump capacity, if utilised 0 required collecting tank capacity, if utilised 0 pipe sizes, and 0 vacuum recovery time.

4.2.1 Pipework design Vacuum systems are designed to operate on two- phase air to liquid flows. The air in the pipework is not, as in a conventional horizontal gravity system, flowing above the wastewater but is entrained into the wastewater where its expansion propels the

wastewater and lowers its bulk density. These factors enable the wastewater to behave more like a gas than a liquid and in particular flow uphill.

The strength of thermoplastics is affected by temperature. In industrial installations where high wastewater temperatures are anticipated, care must be taken in the selection of pipe materials. Wastewater temperatures greater than 70°C should be notified to the designer, so that the design can limit, by pipework design and buffer volume sizing, possible boiling due to pressures lower than atmosphere.

Installation of vacuum pipes and fittings follow current water system practices. Isolation valves are installed in branches and mains to allow portions of the main to be isolated for repairs or troubleshooting.

Although most drainage systems only have relatively short distances between the appliances and vacuum station, long vacuiim pipelines are laid with a series of reforming pockets (see Figure 18). When the propulsion effect of the air has diminished, the wastewater remaining within the pipework will drain under gravity into the reforming pockets. When the next interface valve opens, the movement of the air will remove the water from the pocket and transport it as a slug further towards the vacuum station. This enables the wastewater to be transported to the vacuum station in a series of interface valve operations.

Figure IS - Typical long system pipework layout

Wash hand 60 mm I basins Vacuum ww vucuum line

Typical vacuum pipe installation at

obstruction Obstecle (ductwork.

Pocket line

Vent from

65 mrn vacuum header

Vecuum discharge line

T L Figure 16 - Typical short system pipework layout

4.2.2 System layout (Figures 15 and 16 show examples of long and short system layouts.) The pipes can be tun in lightweight suspended ceilings rather than being cast integrally with the concrete floor slab or floor structure. This allows appliances to be located virtually anywhere in a building.

Air entering the interface units at atmospheric pressure is released from the vacuum drainage system at a suitable point downstream of the vacuum generator.

The risk of water leakage is counteracted by the vacuum prevailing in the pipes, coupled with the fact that the amount of water in the piping system is minimal.

Most of the large vertical ducts for conventional gravity stacks and multiple ventilating pipes are eliminated due to the smaller diameters of vacuum pipework and the reduced ventilation requirements. The pipes are installed in a near horizontal profile, without backfall (0.2-0.596 fall) to a suitably located vertical pipe. Once the vertical pipe (stack) is installed all horizontal pipes may be connected at each level in the building in the void between floor and ceiling, subject to lift height restrictions. All service connections from the interface units could either be lifted to the pipeline in the ceiling above or dropped through the floor to the pipeline below. This makes installation one floor at a time possible, which is particularly valuable in building refurbishment.

Preferably, connections to horizontal pipelines should be arranged so that the branch pipe enters from the top by way of a Y-fitting (see Figure 17). As a minimum it shall connect into the top sector of the vacuum main pipeline contained within the angle of +/- 60" about the vertical axis. Vertical lift piping connecting to horizontal pipelines should enter from the top by way of a Y-fitting. Precautions should be taken, e.g. the use of a check valve suitable for vacuum drainage, to prevent filling the riser with wastewater by back surges. Horizontal piping connecting to vertical stacks should enter by way of single Y-branches. Multiple connections should be at staggered levels where practical.

For a larger building, it is customary to divide the system into smaller sub-systems, possibly with a crossover, if not cost prohibitive, so that, in the event of failure of part of the system, each sub-system could operate as a standby for the other. The crossover pipework would be located between the vacuum station and pipework manifold of each sub-system. An isolation valve, the crossover valve, located within the crossover pipework would, for normal operation, be closed.

4.2.3 Lifts and reforming pockets Generally, when toilets and interface valves are required to discharge into an overhead vacuum pipeline, the lift should be made immediately after each toilet and interface valve. Vertical lifts of 4m

I I n CORRECT] I

lkixiKL slop 0 5% >.?Qm

I Check valve

Sbpe O S %

Standard dser plpe 3m I 1 I I

NOT CORRECT

Figure 17 - Connections must always enter from the top

are common, however greater lifts may be achieved in consultation with the specialist system designer.

In mixed black and grey water systems, a check valve should be fitted on all risers in excess of 2n1 at the connection to the horizontal pipeline.

Pockets can be used to negotiate obstacles such as beams and ducting. If the obstacle is greater than l m in length, then a reforming pocket must be placed at the lift (see the detail in Figure 18).

I I Figure 18 -Pockets

Pipe profile changes or lifts are accomplished by using two 45" elbows joined by a section of pipe - or, for shallow lifts, a single elbow may be used. For efficient use and in situations where the available energy supply is limited, profile changes should be limited, wherever possible to around 300mm or less.

4.2.4 Isolation measures Isolation valves shall be installed in the pipelines in order to permit repairs or to locate faults at branches off the stack connections and where the pipeline exceeds 200m in length.

All near horizontal pipelines should have 0.2-0.5%) fall in the direction of flow, however on long runs this loss of height can be recovered at a reforming pocket, bringing the pipeline back to its original invert level.

4.2.5 Cleaning eyes Cleaning eyes (see Figure 19) are only required where other forms of access are not available. The use of readily demountable fittings and large diameter (>60mm) pipework minimises the need for cleaning eyes. Eyes may be fitted to dead ends of pipes and may be used for points for subsequent expansion of a system. It should be noted that when the pipework is opened for maintenance, and the system is operating, wastewater will not escape. Any liquid in front of the blockage will be evacuated by the vacuum, and as the pipework is open to the atmosphere, the discharge valves will not operate, hence no more liquid will be admitted into that part of the system.

0 estimated repair or replacement times of interface units

0 maintenance schedule for vacuum station equipment

0 procedures for removal or repair of vacuum station equipment and their temporary effects on system performance, if any

vacuum station equipment

temporarily lost or reduced

0 estimated repair or replacement times for

U precaution routines if system performance is

0 training of maintenance personnel 0 recommended stocking of spare parts, and

estimated cost of maintenance per year.

Upper end 01 a ve&d line

End 01 a horizontal line

_____

Figure 19 - Cleaning eyes

4.2.6 Appliance traps The fitting of water seal traps on appliances will depend upon the type of installation. If a low vacuum system is used and the interface unit is adequately ventilated, conventional appliance traps, or self-sealing waste valves, may be installed if the length of pipework between the appliance and the interface valve may retain material, such as urine deposits or vegetable matter, that could decompose and produce odours.

If a high vacuum system is used, or the interface unit is not adequately ventilated, air will be drawn into the system through the waste pipes of the appliances. Such an airflow would displace the water seals of any conventional traps, hence they should not be fitted to such appliances.

4.3 Maintainability System maintainability affects not only maintenance costs but also availability. The following aspects are the minimum that should be addressed as part of system design: [7 fault-finding procedures 0 access to all interface units, isolation valves,

cleaning eyes, check valves and other items that need inspection andlor service

their temporary effect on system performance, if any

0 maintenance schedules for interface units in relation to cycle frequency and endurance

[7 procedures for removal of interface units and

4.3.1 Operation and maintenance manual A detailed operation and maintenance manual (OMM) must be supplied with each system. It should contain detailed instructions on the items shown in the box on page 16 (the frequencies and tasks are examples only).

In addition, the OMM should contain details of: operation and handling special procedures for the vacuum station

0 safety equipment in the vacuum station 0 safety instructions 0 vacuum generators 0 forwarding pumps 0 instrumentation 0 part lists and part numbers 0 assembly drawings 0 layout and wiring diagrams 0 interface valve mechanism and adjustments 0 sensor/controller mechanism and adjustments 0 vacuum toilet flushing mechanism and

0 troubleshooting guide 0 system malfunctions and alarms c] record keeping, and 0 details of equipment suppliers and

manufacturers.

adjustments

4.4 Noise control Normal noise control procedures should be applied to the system, such as securing the pipework and careful direction of the exhausted air from the system. The noise generated by the air entering the interface units can be minimised by appropriate adjustment of the timings and air to water ratios.

Noise characteristics of appliances and pipework fixings may be evaluated using the tests specified in I S 0 5135.

EXAMPLE OPERATION AND MAINTENANCE TOPICS AIUs

Annually - visual inspection, and cleaning of buffer volume, sensor and valve. Check the function of the unit and carry out maintenance if necessary.

Five yearly - remove, dismantle and examine the interface valve and renovate or replace, as appropriate.

Vacuum toilets Annually - inspect, clean and check the function of the toilet and carry out maintenance if necessary. Remove and replace the rinse ring. Five yearly - remove and replace the components of the water inlet valve and the discharge valve. Inspect the functional control flushing mechanism and carry out maintenance if necessary.

Vacuum station Weekly - visual inspections; record hours run by vacuum generators and forwarding pumps (inspect data on chart recorder). Monthly - routine operational maintenance. Annually - mechanical and electrical maintenance.

BIBLIOGRAPHY

Sources of reference: the Environmental Protection Agency (EPA) in the USA intemet sites and manufacturers’ literature.

Standards (Note: single part standards are dated, multiple part standards are not.

International Standards I S 0 5135: 1999 ‘Acoustics. Determination of

sound power levels of noise from air- terminal devices, air-terminal units, dampers and valves by measurement in a reverberation room’

European Standards EN 12109:

EN 1091:

EN 752

1999 ‘Vacuum drainage systems inside buildings’ 1997 ‘Vacuum drainage and sewerage systems outside buildings’ ‘Drainage and sewerage systems outside buildings’ ‘Gravity drainage systems inside buildings’

EN 12056

British Standards BS EN IS0 5135: 1999 ‘Acoustics. Determination of sound

power levels of noise from air-terminal devices, air-terminal units, dampers and valves by measurement in a reverberation room‘ 1999 ‘Vacuum drainage systems inside buildings’ 1997 ‘Vacuum drainage and sewerage systems outside buildings’ ‘Drainage and sewerage systems outside buildings’ ‘Gravity drainage systems inside buildings’ 1985 ’Code of practice for building drainage’

BS EN 12109:

BS EN 1091:

BS EN 752

BS EN 12056: BS 8301:

BS 8005 ‘Sewerage‘

National Regulations The Building Regulations for England and Wales (Parts G and H) The Water Supply (Water Fittings) Regulations The Scottish Water Fittings Byelaws The Building Standards (Scotland) Regulations: Technical Standards (Part M) The Building Regulations (Northern Ireland) Technical Booklet N

BRE Garston Watford WD25 9XX TelOl923 664040

The Chartered Institution of Building Services Engineers (CIBSE) Hornchurch 222 Balham High Road Essex Balham RM12 6NB London SW12 9BS TelOl708 472791 TelO20 8675 5211

Institute of Plumbing 64 Station Lane