How do you select the right steam trap?

Considerations for Selecting Steam Traps

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's 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.

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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 does not allow the trap to operate correctly. Proper system design and attention to steam trapping are paramount.

• Failure to remove condensate from the path of high-velocity steam in the pipework.
• Applications where condensate has to lift to a return line or return to a pressurized system.
• Inability of condensate to properly enter or travel along an undersized return line due to flooding or overpressurization with the throttling effects of flash steam.

Modern design and manufacturing techniques have produced more robust steam traps, allowing them to last longer under normal conditions and withstand the effects of poorly designed systems. However, even the best trap will be less effective and have a shorter working life if installed in a poorly designed system.

If a steam trap persistently fails on an established system due to waterhammer, it is likely the system layout's fault rather than the trap. The solution is to investigate and correct the system inadequacies.

Two important applications are the drainage of steam mains and temperature-controlled heat exchangers. Steam mains should be drained at regular intervals with adequately sized drain pockets, and temperature-controlled heat exchangers must allow condensate to drain freely.

If a lift after the trap is required, it may lead to waterhammer regardless of the trap fitted. In such situations, the trap should be complemented with a pump or changed for a pump-trap.

Proper pipework design and installation help maintain the system's thermal performance throughout its service life.

Dirt

Dirt is another major factor that must be considered when selecting traps. Although steam condenses to distilled water, it can sometimes contain trace products of boiler feed treatment compounds 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, makes them prone to malfunction (due to added friction) or even blockage. The sensor element in such traps can be readily cleaned and is not subject to fouling.

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 the 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 inherently small hole frequently blocks. Enlarging the hole (as 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, transferring the blockage from the orifice trap to the strainer, which requires regular cleaning.

Strainers

These devices are frequently forgotten about in steam systems, often 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 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 it is expected to operate up to. 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 another section.

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. Steam locking is prevented with traps that incorporate a 'steam lock release' valve. This valve allows the steam locked in the syphon pipe to be bled away past the main valve. The float trap is the correct choice on rotating machinery such as drying cylinders.

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

Group trapping

Group trapping describes using one trap serving more than one application. However, this method often leads to inefficiencies, particularly in equipment operating at different pressures. It is generally not good practice to group trap equipment operating at different pressures. When group-trapped processes have separate temperature control, the situation can worsen.

One possible application suitable for group trapping is an air handling unit with multiple heater sections in series, where the heater sections share any load change. Still, the condensate drain connections and common pipework must be generously sized.

Historically, one type of steam trap was used for group trapping, which was large and expensive. Today, steam traps are considerably smaller and more cost-effective, making individual heat exchangers better drained. It is always better for steam-using equipment to be trapped individually.

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 passersby, but the risks can be minimized by reducing the severity of the discharge. This may be achieved by fitting a simple diffuser to the end of the pipe, reducing the ferocity of discharge and sound levels by up to 80%.

Steam Trap Selection: How Application Affects Selection

Steps for Steam Trap Selection

Given the large variety of steam traps and their operating characteristics, users may encounter difficulty in selecting the correct trap to effectively drain condensate from their steam applications.

Key trap selection considerations should include pressure and temperature ratings, discharge capacity, trap type, body material, and other relevant factors. 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 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 the 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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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 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

* 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 is 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