How do you select the right steam trap?

Considerations 

By definition, a steam trap must trap or hold back steam whilst at the same time not restricting the passage of condensate, air, and other incondensable gases. The basic requirements of good steam trapping have already been outlined but it is worth repeating that the performance of the plant is paramount. The trap selection follows on the basis that the requirements of pressure, condensate load and air venting have been met, in the provisional selection. However, system design and maintenance needs will also influence performance and selection. Please refer to the following sub-sections in this Module for further advice on this matter.

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Waterhammer 

Waterhammer is a symptom of a problem in the steam system. This could be due to poor design of the steam and condensate pipework, the use of the wrong type of trap or traps or a leaking steam trap, or a combination of these factors. It is often futile to install the correct trap for an application if the system layout will not allow the trap to operate correctly. It is equally pointless to install the correct layout and not pay proper attention to steam trapping. The Modules 11.6 to 11.11 inclusive 'Selecting steam traps' will deal with the correct matching of steam traps to applications and layouts. The proper layout of steam pipework is also dealt with in Block 10 - 'Steam Distribution'. Symptoms of waterhammer are often attributed to malfunction of the steam trap. A more likely explanation is that a faulty steam trap has been damaged by waterhammer. Waterhammer can be caused in a number of ways, including:- 

• Failure to remove condensate from the path of high velocity steam in the pipework. 
• From an application which is temperature controlled and where condensate has to lift to a return line, or return to a pressurised system. 
• The inability of condensate to properly enter or travel along an undersized return line, due to either (a) flooding, or (b) overpressurisation with the throttling effects of flash steam.

Modern design and manufacturing techniques have produced steam traps which are more robust than those of their predecessors. This allows the steam trap to last longer under normal conditions, and will also be better able to withstand the effects of poorly designed systems. Basically, however well a steam trap is made, if it is installed in a poorly designed system it will be less effective and have a shorter working life.

If a steam trap persistently fails on an established system due to waterhammer, it is probably the fault of the system layout, rather than the trap. The solution is to investigate and eradicate the true cause of the problem by correcting the system inadequacies.

Two important applications are the drainage of steam mains, and of temperature controlled heat exchangers. 

As a general rule, steam mains should be drained at regular intervals of 30 to 50 metres with adequately sized drain pockets. The bottom of any riser must also be drained. 

Temperature controlled heat exchangers can only work effectively if condensate is allowed to drain freely from them. If there is a lift after the trap, there will always be a tendency for waterhammer, whichever trap is fitted. In this situation, the trap should either be complemented with a pump, or changed for a punp-trap . This subject will be dealt with in further detail in Block 13 - 'Condensate Removal' 

It is important that the pipework is designed and installed correctly. This will help to maintain thermal performance of the system throughout its service life. 

Dirt 

Dirt is another major factor which must be considered when selecting traps. Although steam condenses to distilled water, it can sometimes contain trace products of boiler feed treatment compound and natural minerals found in water. Pipe dirt created during installation and the products of corrosion also need to be considered. 

An intermittent blast action trap is the least likely to be affected by dirt. In thermostatic traps this means that the balanced pressure thermostatic trap is preferable, although the larger flat valve associated with some diaphragm traps can cause difficulties. 

The dribbling action of bimetallic traps, coupled with the arrangement of the valve stem passing through the seat, means that these are most prone to malfunction (due to added friction) or even to blockage. It is sometimes claimed that the sensor element can be readily cleaned and is not subject to fouling. However, fouling of the element is rarely a problem: the relevant parts are the 'dynamic clack' valve mechanism, which tends to be self-cleaning due to its positive opening action.

Float-thermostatic steam traps are quite resistant to dirt. As an extreme example, when draining concrete curing autoclaves, the residual sand which precipitates into the condensate can be carried through large float-thermostatic steam traps quite successfully, due to the low velocity flow through a relatively large orifice.

The inverted bucket trap has an air vent hole in the bucket. If this blocks, it can cause the trap to air-bind and be slow to react. If this happens, the scale or dirt blocking the air vent must be dislodged, which requires the trap to be removed from service. 

The impulse trap is intolerant of dirty conditions. The fine clearance between plug and tapered sleeve is susceptible to high velocity flow and the plug will frequently stick in an intermediate position. The trap seizes in a fixed position and will either pass steam or condensate depending on the rate of condensation.

The fixed orifice device is least suited to dirty conditions. The hole is inherently small and frequently blocks. Enlarging the hole (as is sometimes done in desperation) destroys the concept of sizing on a fixed orifice. It is wasteful and in some cases merely delays the time until blockage re-occurs. A strainer is often supplied and fitted but this has to be extremely fine to be effective. 

This simply transfers the blockage from the orifice trap to the strainer, which, in turn, requires regular downtime for cleaning.

Strainers

These devices (Figure 11.5.1) are frequently forgotten about in steam systems, often, it seems, in an effort to reduce installation costs. Pipe scale and dirt can affect control valves and steam traps, and reduce heat transfer rates. It is extremely easy and inexpensive to fit a strainer in a pipe, and the low cost of doing so will pay dividends throughout the life of the installation. Scale and dirt are arrested, and maintenance is usually reduced as a result.

Selection is simple. The strainer material is selected to match the type of installation and the system pressure up to which it is expected to operate. Different filter screen sizes may be considered for differing degrees of protection. The finer the filter, the more often it may need cleaning. One thing is certain, strainers are far easier and cheaper to buy and maintain than control valves or steam traps.

Further information on strainers is given in Block 12 - 'Pipeline Ancillaries

Steam locking

The possibility of steam locking can sometimes be a deciding factor in the selection of steam traps. It can occur whenever a steam trap is fitted remotely from the plant being drained. It can become acute when condensate is removed through a syphon or dip pipe. Figure 11.5.2 illustrates the problem of steam locking in a rotating drying cylinder by using a syphon pipe.

In Figure 11.5.2 (i) the steam pressure is sufficient to lift condensate up the syphon pipe, through the steam trap and away. Figure 11.5.2 (ii) shows what happens when the level of the condensate at the bottom of the cylinder falls below the end of the syphon pipe. Steam enters the syphon pipe and causes the steam trap (in this case a float type) to close.

The trap is temporarily 'steam locked'. Heat loss from the cylinder will result in the formation of more condensate which, as a result, is unable to reach the trap. Figure 11.5.2 (iii) shows the cylinder becoming increasingly waterlogged which will result in a reduced drying rate from the cylinder and an increase in the power required to turn the cylinder. In extreme cases the cylinder may fill to the centre line and damage may then result from mechanical overload

To relieve this problem a trap is needed with a 'steam lock release' valve. This is an internal needle valve which allows the steam locked in the syphon pipe to be bled away past the main valve. The float trap is the only type of trap with this facility and is the correct choice on rotating machinery such as drying cylinders. Because the needle valve is just open enough to avoid steam wastage it has a limited capacity to vent air. Traps of this type are often provided with combined air vents and steam lock release (Figure 11.5.3). The manually operated steam lock release mechanism works independently of the automatic air vent action. A standard float-thermostatic steam trap is shown in Figure 11.5.4

Other types of traps will open and eventually cope with a steam lock, however, the drainage and plant performance will be erratic. This is clearly unacceptable to users of process plant where batch times, quality and efficiency are of high importance.

Group trapping 

Group trapping describes the use of one trap serving more than one application. Figure 11.5.5 shows two batch processes (jacketed pans) operating at two different steam pressures with the drain line from each connected to one steam trap. The higher pressure in plant B will allow condensate from this vessel to drain but will stop condensate being discharged from plant A as check valve C will be held closed. Plant A will waterlog and will suffer a severe drop in performance.

For this reason, group trapping of equipment operating at different pressures is not good practice. But what if equipment operates at the same pressure? Consider the following installation shown in Figure 11.5.6.



In Figure 11.5.6, the content of pan A is almost up to temperature and is condensing relatively little steam. Pans B, C and D have just been filled with cold product and, as the steam is turned on, their condensation rates are much higher than pan A. Consequently, the steam velocity along these supply pipes is much higher, resulting in a higher pressure drop along each of the branch lines. Lower steam pressures will exist at the pan inlets of B, C and D and in their steam jackets, (due to B, C and D having a higher condensing rate than pan A) reducing their heating ability and increasing their production times.

Because of this, the pressures at the drain outlets of pans B, C and D are also lower than that at pan A. Steam will flow from pan A via the condensate drain line to the other pans to equalise the pressures, and the condensate from the other pans will have to flow against this steam flow. When the drainage points of different vessels at different pressures are connected to one trap, the vessel with the highest pressure (in this instance pan A) will impede the flow of condensate from the others. Those vessels with the greatest need to discharge condensate (at this instance pans B, C and D) will tend to waterlog. Hence, the condensate arrangement shown in Figure 11.5.6 is unlikely to be satisfactory. The situation can be aggravated when group-trapped processes have separate temperature control.

One possible application suitable for group trapping is an air handling unit with multiple heater sections in series (Figure 11.5.7).

This 'flow' type application differs from the batch (or non-flow) process in Figure 11.5.6. The heater sections will always share any load change as they are served by the same control valve. It is important that the condensate drain connections and common pipework are generously sized to allow adequate condensate flow in one direction against steam flow in the other. It will only work where all sections are fed by one control valve and the same secondary fluid is being heated by all sections.



The original reason for group trapping was that there used to be only one kind of steam trap. It was the forerunner of the present day bucket trap, and was very large and expensive. Steam traps today are considerably smaller and cost effective, allowing individual heat exchangers to be properly drained. It is always better for steam using equipment to be trapped on an individual basis rather than on a group basis.

In many instances it may be necessary to use a pump-trap on temperature controlled equipment, to remove condensate properly.

Diffusers

With steam traps draining to atmosphere from open ended pipes, it is possible to see the discharge of hot condensate. A certain amount of flash steam will also be present relative to the condensate pressure before the trap. This can present a hazard to passers by, but the risks can be minimised by reducing the severity of the discharge. This may be achieved by fitting a simple diffuser (Figure 11.5.8) to the end of the pipe (Figure 11.5.9) which reduces the ferocity of discharge and sound. Typically, sound levels can be reduced by up to 80%.

 

Steps for Steam Trap Selection

Given the large variety of steam traps and their operating characteristics, users may encounter some difficulty when trying to select the correct trap to most effectively drain condensate from their steam applications.

Key trap selection considerations should include pressure and temperature ratings, discharge capacity, trap type, body material, and many other relevant factors. While it may seem daunting at first, this process can generally be separated into four easy-to-understand steps:

• Step 1: Determine discharge requirements of the steam trap application (e.g. hot or subcooled discharge), and select the matching trap type.
• Step 2: Select trap model according to operating pressure, temperature, orientation, and any other relevant conditions.
• Step 3: Calculate application load requirements and apply the trap manufacturer&#;s recommended safety factor.
• Step 4: Base the final trap selection on lowest Life Cycle Cost (LCC)

The first article of this three-part series will focus on how the steam trap application affects the steam trap selection process.

Steam Trap Applications

Steam traps are usually required to drain condensate from steam piping, steam-using process and comfort heating equipment, tracer lines, and drive-power equipment such as turbines. Each of these applications may require the steam trap to perform a slightly different role.

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Additional resources:
Types of Valves in Cryogenic Application

Different Steam Trap Applications

Steam trap selection depends on the trap application.

For Steam Distribution Piping

The role of steam distribution piping is to reliably supply steam of the highest reasonable quality to the steam-using equipment or tracing lines. One of the most important roles of steam traps on steam piping is to help prevent the occurrence of water hammer. This is done by selecting a trap that is designed to prevent condensate from pooling, which means traps with little to no subcooling of condensate (i.e. rapid near-to-steam temperature discharge) should be chosen.

For Steam-heated Equipment

Because the performance of steam-using process equipment and comfort heating equipment (e.g. air heaters) is directly tied to productivity and product quality, it's important to select a trap that helps shorten start-up time and does not allow condensate to pool into the equipment, causing uneven heating, low heat transfer, and other similar problems. Traps that continuously discharge condensate are typically recommended for these applications.

Such applications may also experience stagnant start-up air left over from condensed steam. As a result, an air venting function is also typically required in the trap to remove air and other non-condensable gases trapped in equipment and adjacent piping.

Also, some steam-heated equipment might experience problems from a modulating steam supply valve (e.g. control valve) that adjusts for heat demand and subsequently lowers the delivered steam pressure below that of the backpressure. When this phenomenon occurs, the condensate flow &#;stalls&#;, and a different type of drainage device is needed. Under stall conditions, a combination pump and trap supplied with a higher secondary pressure is needed to power the condensate discharge through the trap (e.g. PowerTrap®).

For more information on Stall, please read: What is Stall?

For Tracer Lines

Steam traps for tracer lines have different requirements because they are typically used with copper piping (because of its high thermal conductivity) to heat and maintain the fluidity of viscous fluids at temperatures below 100 °C (212 °F). A trap that has been designed to counter blockage from copper precipitate and that can efficiently use the sensible heat of steam/condensate is required.

For Power-drive Equipment

Power-drive equipment includes all turbines used in compressor, pump, or generator applications, but may also include steam hammers or wheels. In each power-drive application, condensate should be removed as quickly as possible for safe and effective operation, and should not pool inside the equipment to prevent damage.

Summary Table of Applications and Steam Trap Requirements

Application Trap Requirements Product Examples Steam Distribution Piping

• Tight seal to minimize steam leakage even with low condensate loads
• Unaffected by environment, even in adverse weather conditions
• Ability to vent start-up air and operating air
• Continuous condensate discharge to minimize pooling
• Unaffected by back pressure in return
• Non-blast discharge characteristics in open-drainage applications

SS / FS Series

Steam-heating Equipment

No Stall

• Continuous condensate discharge to maximize heating consistency and minimize pooling
• Unaffected by large variations in condensate load
• Ability to vent both start-up and operating air
• Ability to discharge condensate even at lowest available differential pressure and operate effectively against high backpressure
• Ability to 'fail open' so that condensate is discharged even if trap is damaged or worn
• Non-blast discharge to minimize piping erosion

JX Series

Steam-heating Equipment

Stall

• Same as above, except;
• No-subcooling condensate discharge from equipment to maximize heating consistency
• Ability to discharge condensate without steam loss regardless of NEGATIVE or POSITIVE differential pressure conditions
• May require other components to discharge condensate if system is damaged or worn

GT Series

Tracer Lines

High Temp.

• Compact and light
• Little to no subcooling
• Trap suitable for operation in all piping orientations
• Requires scale / copper precipitate removal function if frequent blockage

SS series / LV21 / P46S

Tracer Lines

Low Temp.

• Same as above except;
• Subcooling preferred:
  • to use steam&#;s sensible heat
  • to achieve lower temperature

LEX3N

Power-driven Equipment

Positive Pressure

• Tight seal to minimize steam leakage even with very low condensate loads
• Unaffected by environment, even in adverse weather conditions
• Ability to vent start-up air
• Continuous condensate discharge to minimize pooling
• Unaffected by back pressure in return
• Non-blast discharge characteristics in open-drainage applications

JH / FS series

Power-driven Equipment

Negative Pressure

• Same as above, except;
• Ability to discharge condensate generated in vacuum condition
• May require other components to discharge condensate if system is damaged or worn
• System must prevent reversed flow

GT series

* Provided as a general reference. Please consult a steam specialist such as TLV if you are unsure about trap selection or piping design.

After carefully evaluating the steam application discharge requirements and understanding which type of trap is most effective, the next step is matching steam trap specifications with operating conditions.

Operating Conditions Effect on Trap Specifications

System conditions determine the minimum trap specifications for pressure, temperature, discharge capacity, material, and connection type.

Installed Piping and Piping Connections

Installed piping influences connection type and sometimes the trap body material, so it important to make sure that the selected trap meets the piping requirements. For example, a trap may have a standard connection in NPT (national pipe thread), but the piping pressure requires socket weld.

Additionally, other requirements include that the discharge capacity must be suitable for the maximum load at minimum differential pressure under all environmental conditions.

Body Material

Trap body material is one of the first items to look at when selecting a trap. The material is selected based on the maximum operating temperature and pressure at the condensate discharge location (CDL), the surrounding environment, and requirements for longevity/ minimal maintenance. The material must also meet the pressure test and maximum pressure and temperature piping design specifications.

The materials used for the steam trap body, cover, and other pressure-resistant parts are no different from those used in other types of valves. Some examples are:

• Gray Cast Iron/ Ductile Cast Iron
• Carbon Steel
• Stainless Steel

The maximum applicable pressure and temperature of the body material are not necessarily equivalent to the maximum operating pressure and temperature of the trap. This is because the maximum operating pressure and temperature can be limited by the pressure/temperature resistance of other parts such as gaskets and other internal components.

In addition, different standards such as ASME or DIN can affect the maximum operating pressure / temperature of the trap material. For example, A126 cast iron has a maximum allowable pressure of 13 barg (190 psig) according to DIN standards, but 16 barg (250 psig) according to ASME standards. Also, stainless steel traps have recently become more and more popular because they are typically easier to maintain and offer a longer service life.

Sizing

A large number of steam users improperly select trap size based on the size of existing piping. However, trap size should closely match the size of the piping on the outlet side of the equipment that supplies condensate to the trap.

It is generally recommended to size condensate piping on the discharge side of equipment that supplies condensate to the steam trap according to the following table:

Maximum Condensate Load Equipment Outlet Piping Size Less than 200 kg/h [440 lb/h] 15 mm [1/2 in.] 200 - 500 kg/h [440 - lb/h] 20 mm [3/4 in.] 0.5 - 1 t/h 25 mm [1 in.] 1 - 2 t/h 32 mm [1 1/4 in.] 2 - 3 t/h 40 mm [1 1/2 in.] 3 - 5 t/h 50 mm [2 in.] Over 5 t/h 665 - 100 mm [2 1/2 - 4 in.]

* Provided as a general reference. Please consult a steam specialist such as TLV if you are unsure about trap selection or piping design.

Generally, the trap should never be sized smaller than the equipment outlet piping because this can lead to waterlogging and ensuing damage and / or heating problems.

In addition, pipe sizing at the trap outlet should not be based on trap size, but instead should be designed to deliver the required flow rate and limit pressure loss for two-phase flow. For more information on this topic, please read: Condensate Recovery Piping

Connection Type

Most steam users typically require threaded (screwed), socket-welded, or flanged steam trap connections depending on the standard national, industry, or company codes and specifications.

Threaded connections cost much less than flanged connections to install, but need to be screwed-in during installation, meaning that either the trap outlet piping needs to remain disconnected or a union needs to be used to allow for easy trap replacement. On threaded connection steam traps, it is important that the trap threads follow official standards to help minimize poor connection sealing to the connected piping.

Traps with socket weld connections are generally preferred in some plants to limit the amount of steam leaks, but socket weld connections can be more difficult to remove during replacement, and may also have higher installation or maintenance costs. Additionally, some areas may have shortages of qualified welders, which can reduce the overall installation or repair efficiency.

Traps with flanged connections can be easily removed and replaced only if the new trap has the exact same size and face-to-face dimension. It is best to require a strict face-to-face dimension according to a trap manufacturer&#;s standard production item when specifying flanged traps on new construction projects.

Example of Trap with Flanged Connections

After selecting the trap specifications according to operating conditions and environment, the next step is evaluating the necessary discharge capacity that includes the safety factor, and selecting the most economical trap. For more info on these topics, please read part 2.

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