Steam trap monitoring: Early warning defense system
Plant engineers know that their first line of defense in the battle to conserve steam energy is a properly functioning steam trap. Each failed steam trap can be costly, and in larger systems, where there may be hundreds of traps, the cost skyrockets. Regardless of system size, the impact is felt all the way to the profit line of the financial statement. It’s imperative that steam traps be monitored on a regular basis to be certain they are operating properly.
A steam trap with a 5/16-in. orifice blowing steam will cost approximately $19,500 each year (see Table). If there are just five traps this size wasting steam, the loss is more than $97,000 a year. At a 10% profit level, this represents nearly a million dollars in sales just to pay for this wasted energy.
There are four basic types of steam traps: inverted bucket, thermostatic, float and thermostatic, and thermodynamic (disk). Since the operating characteristics of each type are different, understanding how they operate is paramount in determining if they are operating properly. Each type of steam trap has key operational characteristics and unique failure modes. This compounds the challenge to monitor them.
Inverted bucket traps are mechanical traps that operate on the difference in density between steam and water (Fig. 1). Steam entering under the submerged inverted bucket causes the bucket to float and close the discharge valve at the top of the trap body. Condensate entering the trap changes the weight of the bucket. It sinks and operates a valve to discharge condensate. Unlike other mechanical traps, the inverted bucket also vents air and carbon dioxide continuously at steam temperature. Although these traps are noted for their dependability and long service life, they can fail. When they do, the typical failure mode is in the open position, allowing both steam and condensate to blow through.
Thermostatic traps are modulating devices that operate on a temperature differential (Fig. 2). Normally these traps discharge continuously when condensate is present, but can cycle intermittently. Thermostatic traps can fail in either the open or closed position, depending on the design of the bellows. Another failure mode of these traps is progressive wear with increasing steam leakage.
Float and thermostatic traps modulate during condensate removal and have two valves (Fig. 3). A thermostatic element opens one valve to vent accumulated air, while a floating ball controls the condensate discharge valve.
The valve venting the air can fail in either the open or closed position. The valve discharging condensate can fail either open or close and may leak condensate/steam due to valve and seat wear. Because the condensate discharge valve is at the bottom of the trap, the primary cause for the trap to fail open is dirt or scale, which accumulates in the valve and keeps it from seating securely. Typically, the trap failure mode is in the closed position because the float may have ruptured or collapsed.
Thermodynamic (disk) trap has a controlled disk which is a time-delay device that operates on the velocity principle (Fig. 4). These traps operate intermittently and depend on the change in pressure in the chamber where the disk is located. The disk remains open as long as cool condensate is flowing. When steam or flash steam reaches the inlet orifice, velocity of flow increases, pulling the disk toward the seat. An audible slamming of the valve against the seat as it opens and closes can be heard.
If the valve is worn, the sound of steam leaking through can be heard. Cycling rates of 15-20 times/min. indicate some wearing. A rapid machine-gun-like cycle suggests a failed trap that’s blowing steam.
Monitoring Trap Performance
With four different types of traps, each with a uniquely different operational mode, it’s not surprising that there are various methods of testing steam traps and monitoring philosophies. Some plant engineers opt to have one or two maintenance personnel trained to test traps and empower them to monitor the system. They are expected to properly test the traps, check for leaks in pipes and fittings, ensure insulation integrity, and other related responsibilities.
Steam system monitoring can also be a departmental responsibility. One problem with this arrangement is uniformity of test procedures and the priority given to trap monitoring. Generally, a system-wide testing program is recommended at least once a year, but preferably twice a year, and troubleshooting throughout the ”steam-on” seasons. According to Murphy’s Law, a steam trap will fail shortly after being tested and found operating properly, wasting valuable steam for several months.
When faced with a tight labor market and an understaffed maintenance task force, plant engineers often outsource steam trap monitoring responsibilities. There are organizations that assume full responsibility for an entire steam system from the boiler to the condensate return system. Such firms may ”own” the system and charge the plant for steam used. They also assume steam trap monitoring and maintenance responsibilities.
Steam traps can be equipped with integral monitoring sensors and devices. Some have red and green lights that may suggest whether the trap is functioning correctly or if it has failed. Someone needs to walk the steam lines regularly and observe the lights to determine trap performance.
Other steam traps can be equipped with internal sensors that interpret the trap’s operating conditions. Such units are then hardwired to a central data collection system that records the traps’ performance and identifies the failed units on a data printout.
A recent innovation to monitor steam trap performance is a system that uses radio waves to transmit valuable data from the trap to a central computing unit (Fig. 5). The system gathers instantaneous performance data from hundreds of traps simultaneously. Failed steam traps are quickly identified by location and can be promptly repaired or replaced without maintenance personnel physically observing or testing each steam trap in the plant.
This monitoring system consists of a transmitter, which is threaded into the base of an inverted bucket steam trap equipped with a probe connection, a signal repeater (if needed), a receiver, and a programming module. When installed on an inverted bucket steam trap, the monitoring system detects conditions inside the trap, indicating if it is ”OK,” ”blowing-through,” or ”cold.”
A ”cold” signal indicates temperature reduction because the trap is flooded and condensate may be backing up. This is cause for immediate attention, since reduced temperature could result in product loss and process downtime if conditions are not corrected.
In addition to being installed on inverted bucket traps, the transmitter can also be installed on certain models of float and thermostatic traps.
It’s likely that in any given plant, there will be two, three, or four types of steam traps. What is the best method of monitoring these energy-saving devices? Traditional temperature, sound, sight, and advanced electronic methods can be used to simplify and improve monitoring reliability. Sometimes it’s advisable to use more than one method to determine if a trap is functioning properly.
Temperature method: A pyrometer, an infrared testing gun, or heat-sensitive crayons, paints, or stick-on strips that change color when the temperature rises above a predetermined level provide basic information. Measuring the external temperature of the inlet and outlet pipes gives a rough indication of the system’s upstream and downstream pressure, assuming the traps are supplied with saturated steam. Although the technique of measuring temperature differential across the trap is easy, it does not indicate whether the trap is blowing steam and is probably the least reliable testing method.
Sound method: The sound method is more reliable than the temperature method, but requires a trained ear to distinguish a trap discharging normally from one discharging live steam along with condensate. Sound test methods range from placing the metal end of a screwdriver on top of the trap and the other end against an ear, to using sophisticated stethoscopes and probes. Some test units are equipped to print out data. When using this method, it’s important to filter out background noises, such as pumps, other traps, and process equipment, to obtain an accurate reading.
Visual Method: This approach is a good predictor of a trap functioning properly. As long as the steam system has been designed to return condensate to the boiler, this method requires only a pair of keen eyes and a three-way valve, or test valves on both sides of the steam trap. By shutting off the valve to the condensate return line, and allowing the trap to discharge to atmosphere, the person monitoring the trap should be able to decide if the trap is operating properly.
Find them failed? Fix them!
Steam traps are mechanical devices subject to fatigue and failure. When a steam trap fails, there are three options: do nothing, waste steam and adversely affect production; replace it with a new one that is properly sized for the application and discard the failed unit; or rebuild the trap using factory-authorized parts that usually are available as repair kits. It’s often more cost effective to repair steam traps, especially the larger units, than to discard them.
Many traps are repairable, and when one has failed, consider repairing it rather than discarding it. A trap failure on an inverted bucket steam trap is usually associated with the replaceable valve and seat. Once the valve and seat and a few other selected components are replaced, the steam trap should be as good as new. Repairing such a trap is relatively simple.
When repairing a float and thermostatic trap, it may be necessary to replace the float, which is subjected to tremendous forces from freezing and water hammer. Both conditions can cause the float to rupture. It should be replaced when replacing the trap’s valve and seat components.
When maintaining a steam trap system, take the offensive and implement a steam trap monitoring protocol. Implementing such a system helps ensure plant engineers of winning the energy conservation battle.
— Edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, firstname.lastname@example.org
The author is available to answer questions on steam trap monitoring. Mr. French can be reached at 616-273-1415.
Cost of various sized steam leaks at 100 psi
(assuming steam cost of $5/1000 lb)
Size of orifice (in.) Steam wasted per month, lb Total cos tper month, $ Total cost per year, $ 1/2 835,000 4,175.00 50,100.00 7/16 637,000 3,185.00 38,220.00 3/8 470,000 2,350.00 28,200.00 5/16 325,000 1,625.00 19,500.00 1/4 210,000 1,050.00 12,600.00 3/16 117,000 585.00 7,020.00 1/8 52,500 262.50 3,150.00 The steam loss values assume clean, dry steam flowing through a sharp-edged orifice to atmospheric pressure with no condensate present. Condensate would normally reduce these losses due to the flashing effect when a pressure drop is experienced