VARIABLE FLOW SYSTEMS INCORPORATING DIFFERENTIAL PRESSURE CONTROL VALVES

(DPCVs)Andy Lucas - Crane BS&U

Chris Parsloe - Parsloe Consulting Ltd

Crane Co. was founded in 1855 by Richard Teller Crane, who made the following resolution:

“I am resolved to conduct my business in the strictest honesty and fairness; to avoid all deception and trickery; to deal fairly with both customers and competitors; to be liberal and just towards employees; and to put my whole mind upon the business”

Crane Limited was founded in Ipswich in 1919

Crane Building Services & Utilities was created 2009

Water, Air and Steam Safety Valves

Automatic Air Eliminators

including

including

Differential Pressure Control Valves (DPCVs)

ABC

DPCVs hold pressure constant across a varying resistance (such as a 2 port valve). An impulse tube connects upstream of the variable resistance (A) to the top side of a flexible diaphragm. A separate capillary (or internal tube) connects from downstream of the variable resistance (B) to the underside of the diaphragm. As the overall pressure available A-C changes, the DPCV adjusts its position such that the pressure drop A-Bremains constant. This means that the 2 port valve only needs to close against a fixed, pre-set pressure differential

Two Port Control ValvesThis is important because some 2 port control valves are limited in their ability to close against large pressure differentials.

The simplest form of 2 port control valve is the Thermostatic Radiator Valve (TRV). These valves sense the temperature in the room and gradually close when the required temperature is achieved. They usually rely on the expansion and contraction of a wax capsule inside the valve.

TRVs are typically only capable of closing against around 30kPa pressure differential. Hence, left unprotected in a system with pump pressure greater 30kPa, they may struggle to close and can become noisy.

TRV IV

An in-built temperature sensor closes the valve when satisfied

2 Port Control Valves

Heating and cooling coil circuits use more sophisticated 2 port control valves that are operated by electrically powered actuators. This means that the valves can shut off against much larger pressure differentials (200-500kPa, depending on valve size).

However, simply because the valve can close against such a high pressure differential doesn’t mean it is a good idea to let it do so.

Room sensor

Actuator

Cavitation in 2 Port Control Valves

The closure of valves against excessive pressures can still cause noise and, if the static pressure in the system is low enough, there is a risk of cavitation in the valve. Cavitation is the localised vaporisation of a liquid. When the absolute pressure approaches the vapour pressure of the liquid, dissolved air is released and small bubbles of vapour are formed. These bubbles form and then collapse rapidly releasing enormous amounts of energy which can cause damage to metal components such as valves.

Valve characteristic

Percentage openP

erce

ntag

e flo

w ra

te

If good modulating control is required, then the control valve needs to achieve an equal percentage characteristic i.e. a characteristic that mirrors the heat transfer characteristic of the coil. This will ensure that closure of the valve will achieve a steady reduction in heating or cooling output from the coil enabling close control of internal temperature.

Coil characteristic

α = 0.3

α= 1

Valve AuthorityAn equal percentage control valve can be specified, but it will only operate with an equal percentage characteristic if, when fully open, the pressure loss across it represents a significant proportion of the overall pressure loss in the circuit it controls. This relationship p1 / (p1 + p2) in the diagram below is referred to as “valve authority”. Ideally the authority should never be less than 0.3. The graphs below show the effect of varying valve authority on the valve’s characteristic.

p1

% open

% fl

ow

a=

0.5

a = 0.

1

A

BC

A DPCV holds pressure constant between points A and B regardless of changes in pressure between A and C.

In large systems it is usually impossible to select modulating 2 port control valves with an acceptable authority unless there is some form of differential pressure control that limits the pressure differential against which the 2 port valves have to close. The positioning of DPCVs on sub-branches serving downstream 2 port control valves is therefore essential to achieve good control, as well as to avoid noise or cavitation.

DPCVs to Protect Downstream 2 Port Control Valves

The secondary pumps are variable speed.

Branches to each level feed flow return circuits which are themselves broken down into a series of sub-circuits. Each sub-circuit has its own DPCV.

System Layout

1 in 5 of the central control valves on each circuit should also be selected as a constant flow (3 or 4 port valves) with the aim of achieving approximately 80% reduction in flow at minimum load.

End terminal units should be given constant flow (3 or 4 port valves) to ensure that: - there is flow through the pump at minimum load - water treatment chemicals are circulated to extremities- when control valves open, there is a ready supply of hot or cold water in the mains.

Terminal Branches

This will make it possible to select 2 port control valves with acceptable authorities (i.e. >0.3)

Locations of DPCVs to Facilitate 2 Port Valve SelectionThe constant pressure, controlled by the DPCV must not exceed 1.5 times the pressure drop across the end terminal branch Dptb

Branches should be limited to no more than 12 terminal units per DPCV controlled sub-branch.

Dptb

2 Port Valve Selection2 port valves must be sized to achieve an authority of at least 0.3 i.e. p1 divided by p1 plus p2 must be greater than 0.3 where p1 plus p2 is equal to the total loss through the downstream index branch.

p1 + p2 = total loss through downstream index branch

p1 + p2

p1

Commissioning Features Around DPCVs

Pressure test points should be located adjacent to each capillary tube connection so that the pressure controlled constant by the DPCV can be measured and recorded.

A Capillary tube connects each side of the controlled sub-circuit.

A Companion Valve, FODRV, should be located upstream of the DPCV so that the DPCV can be adjusted until the requireddesign flow rate is achieved.

Pump Speed Control

. .

.

.P

ress

ure

Dp

(kP

a)

Flow Rate Q (kg/s) 50% 100%

Maximum load operating point

Typical energy saving 30-40%

Typical energy saving 50-60%Typical energy

saving 80-90%

The pump energy saving achieved is influenced by how pump speed is controlled. Integral pump controllers enable the pump to be controlled to maintain constant pressure or at an arbitrarily selected pressure proportional to the reduction in flow rate. The savings achieved using these methods are not as good as by using remote differential pressure sensors.

Speed control based on constant pump pressure

Speed control based on proportion of flow rate

Speed control based on remote differential pressure sensors

Pump Speed Control

P

PP

Pump speed should be controlled to maintain the minimum specified pressure differentials at all sensors.

Differential pressure sensor across the DPCV controlled index sub-branch. The pump speed should be controlled to maintain the minimum specified pressure differential.

Could the index move to an upstream sub-branch? (Yes if the entire index branch closes down) If so an additional sensor is required here.

If so, an additional differential pressure sensor should be installed here.

Could the index move to the floor below? (Yes if the entire top floor closes down)

Differential pressure sensors should be located across the most remote DPCV controlled sub-branch with additional sensors on any upstream branches that might become the index under part load conditions. Pump speed should be controlled such that the minimum specified pressure differential at each sensor is maintained.

Features Around Differential Pressure Sensors

PTee off connections to the sensor must be at least five diameters downstream of bends or other restrictions.

Isolating valves should be incorporated in pipe connections so that the sensor can be isolated and removed if necessary.

A by-pass with an isolating valve should be included to allow the differential pressure to be checked and zeroed.

Pressure tappings should be included either side of the sensor, so that the sensor can be checked and recalibrated.

Centralised Valve Modules

Pre-assembled centralised valve modules contain all of the valves required to feed a group of terminal units. Flexible multilayer pipe connects from the module to the terminals.

Valve Module

A large bodied strainer provides protection to downstream valves.

A single DPCV ensures a constant pressure across the flow and return manifolds.

Drain cock for back flushing of terminals

A Ball valve on each flow to enable isolation.

Centralised Valve Modules

Commissioning sets on each return to enable flow balancing.

An integral flushing by-pass in accordance with BSRIA guidance

Centralised Valve Modules

CommPacFlow

Management Module