Separators : International Site for Spirax Sarco

8/10/2019 Separators : International Site for Spirax Sarco

1/6

09/10/2014 11:43Separators : International site for Spirax Sarco

Page 1 sur 6http://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/separators.asp

International site for Spirax Sarco Tel: +44 (0)1242 521361Fax: +44 (0)1242 [email protected]://www.SpiraxSarco.com/

'Wet' steam is a major concern in a steam systemas it can cause process and maintenanceproblems, including lower productivity, erosionand corrosion. Separators are designed to

efficiently remove the moisture from steam flow.The application and selection of different types areconsidered here.

Contact Us

The printable version of this page hasnow been replaced byThe Steam and Condensate Loop Book

View the complete collection of SteamEngineering Tutorials

You are here: Home Resources Steam Engineering Tutorials Pipeline Ancillaries Separators

Separators

Wet steam is steam containing a degree of water, and is one of the main concerns in any steam system. Itcan reduce plant productivity and product quality, and can cause damage to most items of plant and

equipment. Whilst careful drainage and trapping can remove most of the water, it will not deal with the waterdroplets suspended in the steam. To remove these suspended water droplets, separators are installed insteam pipelines.

The steam produced in a boiler designed to generate saturated steam is inherently wet. Although the drynessfraction will vary according to the type of boiler, most shell type steam boilers will produce steam with adryness fraction of between 95 and 98%. The water content of the steam produced by the boiler is furtherincreased if priming and carryover occur.

There is always a certain degree of heat loss from the distribution pipe, which causes steam to condense.The condensed water molecules will eventually gravitate towards the bottom of the pipe forming a film ofwater. Steam flowing over this water can raise ripples that can build up into waves. The tips of the waves tendto break off, throwing droplets of condensate into the steam flow.

The presence of water in steam can cause a number of problems:

As water is an extremely effective barrier to heat transfer, its presence can reduce plant productivityand product quality. This can be seen in Figure 12.5.1, which shows the temperature profile across atypical heat exchange surface.

Fig. 12.5.1 Temperature profile across a heat exchange surface

Water droplets travelling at high steam velocities will erode valve seats and fittings, a condition knownas wiredrawing. The water droplets will also increase the amount of corrosion.

Increased scaling of pipework and heating surfaces from the impurities carried in the water droplets.

Erratic operation of control valves and flowmeters.

Failure of valves and flowmeters due to rapid wear or waterhammer.

Pipeline Ancillaries

Isolation Valves - Linear

Movement

Isolation Valves - Rotary

Movement

Check Valves

Strainers

Separators

Gauges, Sight Glasses,

Vacuum Breakers

Related Content

Separators

Separators are designedto gather small waterdroplets from steam &separate them from thepipeflow.

The Steam andCondensate Loop Book

A comprehensive bestpractice guide to savingenergy and optimisingplant performance, thisbook covers all aspects ofsteam and condensatesystems.

Order your copy today

Feature

Home About Us Products & Services Industries & Applications Training Resources Contact
http://www.spiraxsarco.com/resources/steam-book.asphttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/gauges-sight-glasses-vacuum-breakers.asphttp://www.spiraxsarco.com/contact/http://www.spiraxsarco.com/resources/http://www.spiraxsarco.com/training/http://www.spiraxsarco.com/industries/http://www.spiraxsarco.com/products-services/http://www.spiraxsarco.com/about/http://www.spiraxsarco.com/http://www.spiraxsarco.com/resources/steam-book.asphttp://www.spiraxsarco.com/resources/steam-book.asphttp://www.spiraxsarco.com/products-services/products/pipeline-ancillaries/separators-and-insulation-jackets.asphttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/gauges-sight-glasses-vacuum-breakers.asphttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/strainers.asphttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/check-valves.asphttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/isolation-valves-rotary-movement.asphttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/isolation-valves-linear-movement.asphttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries.asphttp://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries.asphttp://www.spiraxsarco.com/resources/steam-engineering-tutorials.asphttp://www.spiraxsarco.com/resources/resources.asphttp://www.spiraxsarco.com/http://www.spiraxsarco.com/resources/steam-engineering-tutorials.asphttp://www.spiraxsarco.com/resources/steam-book.asp


2/6



Although there are a number of different designs of separator, they all attempt to remove the moisture thatremains suspended in the steam flow, which cannot be removed by drainage and steam trapping. There arethree types of separator in common use in steam systems:

Baffle type -A baffle or vane type separator consists of a number of baffle plates, which cause theflow to change direction a number of times as it passes through the separator body. The suspendedwater droplets have a greater mass and a greater inertia than the steam; thus, when there is achange in flow direction, the dry steam flows around the baffles and the water droplets collect on thebaffles. Furthermore, as the separator has a large cross-sectional area, there is a resulting reductionin the speed of the fluid. This reduces the kinetic energy of the water droplets, and most of them will

fall out of suspension. The condensate collects in the bottom of the separator, where it is drainedaway through a steam trap.

Fig. 12.5.2 A baffle type separator

Cyclonic type - The cyclonic or centrifugal type separator uses a series of fins to generate high-speed cyclonic flow. The velocity of the steam causes it to swirl around the body of the separator,throwing the heavier, suspended water to the wall, where it drains down to a steam trap installedunder the unit.

Fig. 12.5.3 A cyclonic type separator


3/6



Coalescence type - Coalescence type separators provide an obstruction in the steam path. Theobstruction is typically a wire mesh pad (sometimes referred to as a demister pad), upon which watermolecules become entrapped. These water molecules tend to coalesce, producing droplets that aretoo large to be carried further by the gas system. As the size of the droplets increases, they becometoo heavy and ultimately fall into the bottom of the separator.

It is common to find separators, which combine both coalescence and cyclonic type operations. Bycombining the two methods, the overall efficiency of the separator is improved.

Fig. 12.5.4 A coalescence type separator

Separator efficiency is a measure of the weight of the water separated out in proportion to the total weight ofthe water carried in by the steam. Outside the laboratory, it is difficult to establish the exact efficiency of a

separator, as it depends on the inlet dryness fraction, the fluid velocity and the flow pattern. Erosion of pipebends, wiredrawing, and waterhammer are, however, indications of the presence of wet steam in steampipes.

One of the main differences in performance between the baffle type and the cyclonic and coalescence typesof separators is that the baffle type is capable of maintaining a high level of efficiency over a wider pipelinevelocity range. Cyclone and coalescence type separators typically exhibit efficiencies of 98% at velocities ofup to 13 m/s, but this falls off sharply, and at 25 m/s, the efficiency is typically around 50%, according toUniversity research in the UK.

This research has also proven that, for a baffle type separator, the efficiency remains close to 100% over arange of 10 m/s to 30 m/s . The conclusion is that, the baffle type separator is more suited to steamapplications, where there is usually some degree of velocity fluctuation. Furthermore, wet steam will be foundto run at velocities of over 30 m/s if the pipework is undersized.

One method of overcoming this problem is to use a larger size separator and by increasing the diameter ofthe pipework immediately upstream of the separator. This will have the effect of reducing the velocity of thesteam before it enters the separator.

Example 12.5.1If a separator with an efficiency of 90% is fitted to a steam main containing steam with a dryness fraction of0.95, what would the downstream dryness fraction be?

If the initial dryness fraction is 0.95, every kilogram (1000 g) of steam contains:

Since the efficiency of the separator is 90%, only 0.90 x 50 g = 45 g of the water present is removed. Thismeans that the dryness fraction becomes:


4/6



In practical terms, the steam can be considered completely dry.

If however, the separator efficiency is only 50%, only 25 g of the water will be removed. This results in adryness fraction of:

Although an improvement on the original dryness of 0.95, the steam will still contain a significant amount ofwater.

The pressure drop across a baffle type separator is very low due to the reduction in the velocity of the steam,which is created by the large increase in cross-sectional area provided by the separator body. The pressuredrop is typically less than the equivalent length of the same nominal diameter pipe. In comparison, thepressure drop across a cyclonic type separator is somewhat higher, as the velocity of the fluid has to bemaintained to generate the cyclone effect.

On non-critical applications, baffle type separators are typically sized according to the pipeline size; it is

necessary however to check that the chosen size ensures maximum separation efficiency, and that thepressure drop is within acceptable limits. On critical applications, it is more common to select the separatorbased on operating pressure and flowrate, so as to give a suitable efficiency and pressure drop. Sizing acyclonic type separator is more complicated, as it is important to ensure that the velocity through theseparator is suitable to maintain a high level of efficiency and that the pressure drop across the separator isacceptable.

Example 12.5.2 outlines the selection of a baffle type separator from a typical manufacture's specificationchart.

Example 12.5.2Using the sizing chart in Figure 12.5.5, select a suitably sized separator for a pressure reducing station, withan upstream pressure of 12 bar g and passing 500 kg/h of steam through a 32 mm pipeline, If the flowratewere doubled to 1000 kg/h, what size should the separator be?

1. Plot point Awhere the steam pressure and the flowrate cross and draw a horizontal line across from thispoint. Any separator curve that is bisected by this line within the shaded area will operate at near 100%efficiency.

2.Select the line size separator, i.e. 32 mm at point B.

3.The line velocity for any size can be determined by dropping a vertical line from this intersection. Frompoint B, this line crosses the velocity axis at 18 m/s.

4. To determine the pressure drop across the separator, where the vertical line, extended from point B,crosses the line C-C, plot a horizontal line. Then drop a vertical line from point A. The point of intersection, D,is the pressure drop across the separator.

5.Repeating this procedure for a 1000 kg/h flowrate, generates points X, Y and Z. It can be seen that pointYfalls outside the shaded region and the separator will not operate at maximum efficiency. Here, it would beadvisable to use a larger size separator; a DN40 separator would be selected, as depicted by point Z, alongwith a pressure drop of about 0.07 bar at point W.


5/6



Fig. 12.5.5 Manufacturers sizing chart for a baffle type separator

Table 12.5.1 summarises the important differences in the performance of baffle and cyclone type separators.

A suitable steam trap should be fitted to the condensate outlet of the separator to ensure the efficient removalof condensate, without the loss of live steam. The most suitable type of steam trap is the ball float type, whichensures immediate condensate removal. Some separators include the steam trap mechanism inside theseparator body.

Most vertical separators have a tapping on the top of the body. This can be used for an air vent, facilitatingthe removal of air from the steam space during start-up.

InsulationIf a separator is left uninsulated, it can actually induce water droplets to form rather than eliminating them,because of the large surface area exposed to the environment. Furthermore, significant amounts of heatenergy can be lost from the surface of the separator. For example, insulating a separator containing steam at150C and exposed to ambient temperatures of 15C, will produce an annual energy saving of 8600 MJ(Based on heat loss due to radiation only, assuming still air conditions and 8760 hours of operation per year).By fitting an insulation jacket, this heat loss can be drastically reduced and the energy savings justify the

initial cost of the insulation, within an extremely short time.

Insulation jackets designed to fit over a particular separator should be used, as the shape of the separator,


6/6



The printable version of this page has now been replaced by The Steam and Condensate Loop Book

View the complete collection of Steam Engineering Tutorials

Contact Us

particularly if it is flanged, makes it difficult to insulate. Standard flange covers leave the body exposed, andtherefore have a limited effect in the reduction of heat loss.

Even with the best insulation, it is not possible to eliminate all the heat loss from a product. The efficiency ofseparator insulation is typically above 90%. It is important to use a jacket that is designed for a particularseparator; otherwise, the insulation efficiency will decrease. Properly insulated separators also reduce therisk of personal injury from burns.

Fig. 12.5.6 A horizontal separator and insulating jacket

What do I do now?
http://www.spiraxsarco.com/resources/steam-engineering-tutorials.asphttp://www.spiraxsarco.com/resources/steam-book.asp

Documents

Separators : International Site for Spirax Sarco