Research and Development

Steam Trap Selection (How Application Affects 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.

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.

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.

(Source: TLV International Research & Development)

Steam Pressure Reducing Valves

In a steam-using plant, steam is often generated at high pressures and reduced locally to provide heat for each steam user. This is usually done to minimize the diameter of steam distribution piping and enable more cost-efficient steam delivery.

A common way of lowering pressure is by throttling down the size of the steam passageway. For the most basic pressure reduction, it is possible to simply use a conventional globe valve in a fixed partly-open position, or by inserting an orifice plate into the flow of steam. However, any fluctuation in flow rate would be accompanied by a corresponding fluctuation in pressure. To avoid such circumstances, pressure reducing valves (PRVs) can be used to provide precise control of downstream pressure. They automatically adjust the amount of valve opening to allow the pressure to remain unchanged even when the flow rate fluctuates.

While it is possible to maintain a constant pressure by using the combination of an actuated control valve, a pressure sensor, and a controller, a pressure reducing valve offers the advantage of being able to control pressure through fully-automatic self-contained operation, requiring no type of external power source. It can offer the further advantage of extremely rapid response action by immediately sensing and adjusting based on the downstream pressure.

In pressure reducing valves, the mechanism that automatically adjusts the downstream pressure typically uses the balance of forces between the steam pressure and an adjustment spring. At present, this is a universal concept on almost all manufactured pressure reducing valves. However, there are two different ways in which this mechanism is implemented to control the amount of valve opening:

• Non-piloted, Direct Acting Valve: Adjustment spring places downward pressure setting force directly on the main valve.
• Pilot-Operated Valve: Adjustment spring places downward pressure setting force directly on a pilot valve, which is smaller and different from the main valve.

Direct Acting (Non-piloted)

Used for small loads where extremely close pressure control is not needed.

• Pros: Compact size, low price, easy to install.
• Cons: Higher droop (variation from set pressure) than Pilot-operated PRV.

In direct acting pressure reducing valves, the amount of valve opening is determined directly by the movement of the adjustment spring. If the spring is compressed, it creates an opening force on the valve which increases flow. As pressure builds downstream, equalizing occurs by feeding the downstream pressure to the underside of the adjustment spring (usually against a bellows or diaphragm) where its upward force counter-balances against the spring compression. Spring compressive force which opens the valve is limited to allow sufficient spring sensitivity to equalize with downstream pressure changes. The net result is simple pressure control through a valve orifice where high flow rates can cause pressure droop.

Pilot-Operated

Used for larger loads where close pressure control is required

• Pros: Close pressure control, fast response to load variation, may be used across a broader range of flow rates than the direct acting types.
• Cons: Larger size, higher price.

In pilot-operated pressure reducing valves, a pilot valve is used to load a piston or diaphragm that increases the downward force used to open a larger main valve. This enables larger flow capacity with a lower pressure offset (droop). The opening and closing of the pilot valve is controlled by the balance of force between the adjustment spring and the secondary pressure in the same manner that a direct-acting valve operates. However, in a pilot-operated PRV, this opening and closing of the pilot valve purposely delivers pressure to the main valve piston or diaphragm. This pilot flow pressure then causes a downward force that is amplified by the area of the piston or diaphragm to enable opening of a much larger main valve, which in turn provides the ability for very high flow rates.

Because the downward force is amplified through the use of a piston or diaphragm, a small change in the opening on the pilot valve can result in a large change in flow and downstream pressure through the main valve. As a result, there is little change needed in adjustment spring force on the pilot to accomplish quick response over a wide range of steam flow rates. Quick response and tight delivered pressure control represent the main advantages of this type of valve over the direct-acting type.

Summary

From the above characteristics, it can be seen that the function and applications of non-piloted direct acting PRVs differ substantially from those of pilot-operated PRVs.

In short:

• Direct-operated valves are used when loads are small and some downstream pressure droop may be accepted. They are generally used in light load services.
• Pilot-operated pressure reducing valves can respond quickly to varying load conditions while maintaining stable secondary pressure where precise pressure control is needed. They are generally intended for larger load applications.

Typical Applications in a Steam-Using Plant:

• Small load applications such as sterilizers, unit heaters, humidifiers, and small process equipment may typically use a simple Direct Acting PRV for pressure reduction.
• In case of larger flows, such as steam distribution piping, loads may fluctuate greatly depending on the operational status of the recipient equipment. Such load variations and large capacity would call for the use of a Pilot-operated PRV to reduce pressure.
• Furthermore, the amount of steam used by certain equipment at start-up may differ significantly from the amount required during normal operation. Such wide variations may also necessitate the use of a Pilot-operated PRV for pressure reduction.

Source: TLV International, Research & Development Cell

Real Cost of Steam Trap Failures

Finding out failed steam traps and fixing them up can avoid the wastage of steam and improve the productivity and performance of the steam system. It would be worthwhile to understand the implications of steam trap failures on the performance of the steam system.

Simply put, steam traps fail in two ways: open and closed. When a steam trap fails open, it acts like an open valve, completely leaking the steam and condensate. When a steam trap fails closed, it acts like a closed valve not allowing the steam and condensate to pass through it. Whether a steam trap fails open or closed, it does equal damage to the steam system and hence, should be repaired or replaced.

Failure detection is easy if a steam trap fails open and it discharges into the open. If a stem trap fails closed or if a steam trap from which condensate is being recovered fails open, the detection becomes difficult. Hence, it is always advisable to use correct equipment to monitor the steam trap performance on a regular basis.

If a steam trap fails closed, following are the different problems associated with the failure.

Presence of water i.e. condensate in the steam system results in a phenomenon called as water hammer. Condensate, if not removed by a steam trap accumulates at the bottom of the steam lines. In such situations, a bi-phase flow exists in the piping. During such bi-phase flows, condensate is dragged by the steam to travel along with the steam at speeds much higher than the maximum allowable speed for condensate. This results in slogs of condensate travelling at very high velocity in the steam lines. When this slug hits some bend or some equipment, a pressure wave is generated which is called as water hammer. Fail closed steam traps are the major reason for water hammer.

When a steam trap installed on a process equipment fails closed, it stops removing condensate from the process. As a result, condensate accumulation takes place inside the process vessel. Water logging has multiple disadvantages with respect to the performance of the process. First of all, the product which is surrounded by water won’t get heated. This results in non-uniform heating of the product. Water logging will also increase the time required to heat the product to the desired temperature.

Condensate is pressurized fluid. If condensate remains in the steam system and there is a sudden drop of pressure, it will flash and will expand in its volume. Such sudden expansion can damage the piping. Water hammer as well as moisture entrained in the piping can damage various equipment fitted on the line.

When a steam trap fails open, it simply acts like an open valve and hence, a lot of steam would leak through it. As a result, the overall steam consumption of the plant will go up, reducing the efficiency of the operation.

If a leaking trap is discharging to the drain, the entire steam and hence the heat energy that is being leaked, will be wasted. This also increases the boiler load. Many times, it has been observed that a same size of boiler can be used to heat more processes if all the pipe leaks and leaking steam traps are repaired or replaced.

(Source: Forbes Marshall)

Why Steam Traps Fail?

Authors: Bruce Gorelick, Enercheck Systems and Alan Bandes, UE Systems, Inc.

Properly functioning steam traps open to release condensate and automatically close when steam is present. Failed traps waste fuel, reduce efficiency, increase production costs and compromise the overall integrity of the steam and condensate systems. Traps should be tested on a regular basis — or the neglect may be quite costly.

There are three general conditions, which adversely affect traps:

1. Dirt – by far the leading cause of failure resulting in either a leaking or plugged trap.
2. Pressure surges (due to sudden steam valve openings, improper piping, or trap misapplications) resulting in water-hammer and subsequent damage to the internal steam trap components.
3. Over-sizing IB traps can lose their prime; TD traps can experience rapid cycling.

How do we keep problems to a minimum and keep energy costs in check? One simple way is to look for warning signs. Let’s review the most evident signs that should signal a distress call from your steam system.

1. The once lazy plume from your condensate stacks is now an out of control freight train. The steam that is standing at attention from your stack, like a soldier standing at attention, is costing you dearly.
2. Condensate back pressures that have slowly been rising have been causing your electric condensate pump to self destruct. High temperature condensate cannot be handled by conventional electric pumps. Temperatures over 212 degrees Fahrenheit cause conventional electric condensate return pumps to cavitate. Motors burn out and mechanical seals begin to leak when steam is present.
3. Pressure reducing valves (PRVs) or control valves fail to maintain set pressures. Fully or partially plugged traps prevent condensate from being eliminated from the steam space. Un-drained condensate at PRV stations will back up into the steam line and will wiredraw the heads and seats of the reducing valves. Wiredrawing is when high velocity water in the steam system cuts (scores) the surfaces of heads and seats. Even small microscopic cuts will prevent the proper operation of these valves.
4. A production capability has been reduced. Open or closed traps that have failed will negatively impact production. Plugged traps will back condensate up into the process and dramatically reduce system efficiency. Blowing and leaking traps are costly to production due the added and unnecessary energy consumption.
5. Pipe wall thickness of the condensate system has become an issue. Fully open or partially opened steam traps that are not repaired in a timely manner will deteriorate the condensate return piping. Some of the early warning signs begin with steam leaks.
6. The cost to maintain heat exchanger bundles, humidifiers, HVAC coils and other equipment has dramatically increased. Failed traps will prevent proper operation of sensitive equipment. When steam traps fail in a closed position, over time, the stagnant condensate will turn to carbonic acid (co3). Carbonic acid will deteriorate all the metal it comes in contact with. Beyond increased energy consumption, failed open traps will also cause control and efficiency issues.
7. Water hammer can develop in neglected or mismanaged steam and condensate systems. Water hammer literally sounds like someone is hitting a pipe with a hammer. In some cases water hammer can occur when a portion of the steam condenses into water within steam piping. Left un-drained, condensate will spill into the steam system and begin to accumulate. Eventually a wave of water will be created. This slug of water can be carried at high velocity until it reaches an obstruction like a closed valve, a lower elevation, or a sudden change of direction. A trap that is blowing steam can also cause water hammer. Blowing traps create back pressure in the condensate system piping. If condensate piping is already undersized, the problem will be compounded by the additional pressures found by the faulty traps. Un-drained condensate can back up into the steam distribution piping. From the standpoint of plant safety, it is essential to test and maintain the steam trap population. Type “steam water hammer accidents and fatalities” into a search engine; the results should be convincing enough to create an immediate action plan.

The Action Plan

• Perform a regularly scheduled steam trap survey.
• Identify system design issues.
• Perform an insulation audit. Areas where insulation has been removed and never replaced will significantly add to your overall steam production costs.
• Using ultrasound detection equipment, test bypass valves if they exist in your steam system. They may be leaking through when they ought to be shut.
• Turn off seasonally operated equipment such as unit heaters.
• Periodically test control valves or shut off valves in the HVAC system with a hand-held IR temperature instrument and an ultrasonic leak detector. If they are even partially leaking through, they are adding to overall energy costs.
• Audit the system and unused inventory equipment. Remove defunct systems.
• Whenever possible and practical, use a computerized system to control and monitor processes.

Purchase Proper Test Equipment

Even if outside technical professionals are contracted to test the steam system, from time to time, a problem will occur. Time is money. Having the proper equipment and just one trained employee can avoid costly downtime. The two basic pieces of equipment to own are an infrared thermometer and an airborne ultrasonic instrument. Such equipment is readily available in all price ranges. A thermometer with simple features is fine; however, an ultrasonic listening device should be selected more carefully and must have clear signal quality.

This is similar to purchasing an audio system for your home and comparing one set of speakers with another set. When listening to a quality speaker system the nuances of what was actually recorded opens your ears to new level of listening. The same is true of listening to a steam trap. With a fine instrument you can even hear the snap crackle- pop of steam passing across the head and seat of a wiredrawn inverted bucket trap. After all, “hearing is believing.”