User’s view of steam traps
Steam traps are a pain in the neck to maintain. They are difficult to troubleshoot, numerous, geographically widespread, and often inaccessible. Since the energy wasted by a trap is usually unseen, it is tempting to ignore maintenance. Unfortunately, neglecting faulty traps is terribly expensive.
Trap manufacturers are a terrific source of information, and we depend on them for proper application of their products. But plants seldom have steam components from a single supplier and advice between manufacturers is sometimes inconsistent. There are times when users of steam traps must help themselves. This article is written from a user’s point of view.
The main purpose of a steam trap is to discharge condensate without releasing steam. There are four main types of steam traps that accomplish this in different ways.
This trap opens a valve based on temperature. At saturation temperature the trap’s valve is closed. It gradually opens as the temperature goes below saturation temperature. Condensate from a process flows toward the trap and forms a column of water in the pipe ahead of the trap.
The column top surface is at saturation temperature and the bottom at a somewhat lower temperature. Since trap operation is based on temperature, it is effectively controlling the height of the condensate column. The trap’s valve is at the bottom of the column, so only condensate is released (Fig. 1).
This trap will operate under relatively broad pressure and load conditions (see chart, below left). It has excellent startup characteristics, since its valve is fully open at low startup temperatures.
Float and thermostatic (F/T)
An F/T trap has both a thermostatic element and a float. During normal operation, the float-operated valve discharges condensate to maintain a constant water level inside the trap, usually resulting in a continuous, modulated discharge. The thermostatic element will discharge at temperatures lower than saturation, which is useful at startup (Fig. 2).
This trap has a little less operating range than a thermostatic trap. Its capacity is dictated by the size of the float-operated valve orifice. The orifice size also affects the force balance between the float and valve. This balance necessitates a design compromise between maximum flow rate and maximum operating pressure.
For a given body size, a high-flow trap will have a low maximum operating pressure. If the pressure exceeds the maximum allowable, the valve is held closed and the trap does not release condensate.
Bucket traps intermittently discharge condensate that is stored in piping just ahead of the trap. The mechanism of this trap acts like a steam clock, which fully opens the trap’s valve several times per minute. When the valve is opened, condensate is discharged and the valve closes fully (Fig. 3).
This trap has the narrowest operating range of the four main trap types. Its maximum pressure is limited, as with the F/T trap, but it also has a mechanism dictating minimum operating pressure. If pressure falls below the lower limit, the trap’s valve will not close and steam will be released.
Disk traps (also called thermodynamic traps) discharge intermittently using a timing mechanism that opens and closes the trap’s valve as in a bucket trap. It discharges accumulated condensate in the piping just ahead of the trap then closes fully (Fig. 4).
The disk trap’s mechanism does not create the high-pressure limitation that other traps have and is often used for superheated steam service. This trap has the widest operating range of the four trap types. Historically, disk traps do not vent air very well but some new designs do vent air under startup conditions.
Manufacturers recommend traps based on application. Oddly, manufacturers recommend different trap types for the same application. For example, nearly all steam systems have drip traps for distribution piping. Different manufacturers recommend all of the four main steam trap types for this application.
Generally, thermostatic and F/T traps discharge continuously and bucket and disk traps discharge intermittently.
Traps that discharge intermittently must store condensate in the piping just ahead of the trap, which must be taken into account when installing the trap. The amount of storage required is proportional to the load.
For example, if condensate flows into a vertical, 1/2-in. pipe at a rate of 100 lb/hr and none is allowed to exit, the water level will rise 2.8 in./sec. If a trap at the base of the pipe discharges every 15 sec, 42 in. of condensate will accumulate before discharge. If there isn’t at least 42 in. of pipe between the process and the trap, condensate will begin flooding the process. If providing adequate pipe length is not practical, the pipe’s diameter must be increased (see table).
Testing for failure is fairly easy for intermittent discharge traps. If flow is distinctly on then off, the trap is usually functioning. Intermittent discharge can cause a momentary drop in pressure, which can upset tight control loops. The discharge noise from these traps can be objectionable in quiet environments.
The life of a continuously discharging trap can be shortened when subjected to light loads. Thermostatic traps normally discharge continuously, but, as do intermittent traps, they store condensate in the piping ahead of the trap, which must be accounted for in piping design.
The amount of pipe storage required to prevent process flooding can be estimated from the table. It is assumed condensate flows through an uninsulated steel pipe at 100 lb/hr to a continuously discharging thermostatic trap designed for 11 deg F of subcooling. (Note: Pipe length is directly proportional to condensate flow rate.)
Traps are sometimes selected because of how they operate in undesirable environments. For example, some traps handle air (noncondensable gases) in steam better than others do. Selection of a trap on this basis may solve the problem for the trap, but it does nothing to solve the high-corrosion problem that air causes in the rest of steam system.
Generally, it is better to solve the undesirable problem rather than select a trap to tolerate it. Other undesirable factors are water hammer, poor water treatment, and excessively pressurized condensate return piping.
Traps use a small amount of energy to operate. The real expense is in trap life. Many traps fail open; they allow steam to flow when it shouldn’t. Since this type of failure usually does not affect process temperatures, it is often unnoticed.
The wasted steam from a single faulty trap can easily cost more than $100/mth. To achieve energy efficiency, care should be taken to select and install traps that maximize trap life.
Proper selection of a trap requires knowledge of normal pressure and flow. The trap has to be big enough to handle the flow and be rated for the maximum expected pressure. It is common practice to multiply the normal flow rate by a safety factor (which is often two, three, or even higher) in selecting a trap’s flow capacity. This factor accounts for unusual conditions at startup. If the flow rate under the most extreme condition is known, use a smaller safety factor.
An overly large trap will be lightly loaded, which can shorten its life. Avoid using a trap that will normally operate below its minimum flow capacity. Over-sizing may be indicated if traps that normally discharge continuously (thermostatic and F/T) discharge intermittently. Be certain the trap’s minimum operating pressure is low enough to handle operating conditions.
Some situations expose traps to both high and low pressure and flow rates. This is common when a modulating control valve throttles input steam or if the heating load varies widely. In this case, a trap must be selected that has a wide operating range.
A secondary benefit of a wide operating range is universality; these traps can be used for many different applications within a facility. This reduces the number of different traps stocked as spares and reduces the likelihood of errors during replacement.
In some situations, condensate is shared among multiple traps; for example, when draining headers with drip traps. Installing too many traps can create an undesirably light load for each of the traps.
Choose traps that have strengths that are consistent with local conditions. Size them properly and make sure they can be effectively tested and maintained. Install them as recommended by the manufacturer. Following these basic rules goes a long way to reduce steam trap maintenance and cut expenses.
Table of trap pipe storage
|Pipe size, in||Rise rate, in./sec||15-sec rise, in.|
|Rise rate and time are directly proportional to condensate flow rate.|
The author is available to answer questions about steam traps. Mr. Lauber can be reached at 704-587-6481. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, firstname.lastname@example.org .
Install traps as described by the manufacturer. In the absence of more-specific instructions, here are a few basic rules:
Mount the trap below the equipment outlet so condensate flows by gravity to the trap.
Avoid long horizontal pipe sections ahead of the trap (at least 1% downward pitch if longer than a few feet).
Use a strainer ahead of the trap if one is not integral with the trap (orient the strainer so the basket is horizontal).
Use a check valve after the trap if condensate flows upward after exiting the trap.
If a check valve is used with a bucket or disk trap, locate it at least 3 ft downstream of the trap to prevent check valve damage.
Mount the check valve upstream of a bucket trap if pressure varies rapidly.
Do not insulate the trap (except for F/T traps).
Do not insulate at least 3 ft of piping just ahead of a thermostatic trap.
Allow for adequate condensate storage in piping ahead of thermostatic, bucket, and disk traps.
Excellent at cold startup
Easy to diagnose
Widest operating range
Best for light loads
Suitable for superheated steam
Lowest purchase price
Float and thermostatic
Excellent at cold startup
Least sensitive to installation variations
Needs no upstream storage
Lowest working pressure
Easy to diagnose
Suitable for superheated steam
Factors affecting trap life
The life of traps varies widely. Identical traps can have a long life in one installation and fail quickly elsewhere. This is often true even though trap selection was done by the book. Why?
Selection of overly large traps is a common problem. Manufacturers point out a trap’s maximum flow capacity but seldom discuss minimum allowable flow. This is critical for bucket traps, which can hang open if too lightly loaded. Bucket traps should not operate below 15% of capacity at expected operating pressure. Continuously discharging traps, F/T and thermostatic, have problems with light loads also. These traps open their valves in proportion to load; under light load their valves are only slightly open. This can damage the valve and downstream flow passages.
A rule-of-thumb from the control valve industry is never operate a valve less than 10% open. This is a good idea when sizing continuous discharge traps as well. Size these traps so the normal condensate flow rate is no less than 10% of capacity at the expected operating pressure.
The general operating environment can affect trap life. Expect shorter life when operating at high pressure. Presence of non-condensable gases is hard on both traps and piping.
Diagnosing a steam trap’s health is difficult; even experienced technicians have trouble. The most common ways are temperature measurement, ultrasonic sound measurement, and observation of actual discharge. None are foolproof because each measurement must be interpreted.
Traps are faulty if they fail to pass condensate or allow live steam to pass. It can be difficult to tell the difference between the proper flow of flash steam and the improper flow of live steam. This is true with all of the above test methods, including direct observation.
Traps that discharge intermittently (bucket and disk) are fairly easy to diagnose. If the trap’s valve opens then closes without leaking, the trap is working properly. This is readily determined with either the ultrasonic sound or direct observation method.