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'Trap' Steam with Regular Inspections and Yield Big Savings

Key Points
  • Energy loss from failed steam traps can be substantial.
  • Leaking traps should account for less than 5% of the trap population.
  • There are four basic ways to test steam traps: temperature, acoustic, visual, and electronic.

Steam traps are automatic valves found on the steam distribution lines of a boiler heating system. Their purpose is to keep a steam system operating optimally by purging condensate and maintaining steam quality. The devices typically have a float to trap the condensate. Once the steam has condensed to become hot water, it is removed by the trap and is either returned to the boiler or discharged to the atmosphere. The energy-efficient practice is to feed this hot water into a holding tank so that it can be fed back to the boiler.

Source: U.S. Department of Energy
boiler
In steam systems that have not been maintained for 3 to 5 years, between 15% and 30% of the installed steam traps may have failed—thus allowing live steam to escape into the condensate return system. Given that steam systems can include several hundred traps, the energy losses can be substantial. Steam trap replacement can pay for itself in as little as three months, as indicated in examples below. In systems with a regularly scheduled maintenance program, leaking traps should account for less than 5% of the trap population. For those facilities that do not currently have scheduled maintenance, a yearly steam trap audit program should save between 5% to 15% of input fuel.

Potential Savings Example

In a plant where the value of steam is $8.50 per thousand pounds ($/1,000 lb), an open inspection program indicates that a trap on a 150 psig steam line is stuck open. The trap orifice is 1/8 inch in diameter and has an estimated steam loss of 75.8 lb/hr per the table below. By repairing the failed trap, annual savings are as follows:

Cost Savings = 75.8 lb/hr x 8,760 hr/yr x $8.50/1,000 lb = $5,644/yr

If the newly installed trap costs in the range of $600, then the payback is less than two months. A rule of thumb states that if there has been no steam trap survey or maintenance program, upwards of 50% of a system’s traps can be blowing live steam. If a survey is performed annually, this figure drops to about 25%. A bi-annual survey will reduce this even further to less than 12%. For a facility that has a number of steam traps, the annual savings from a steam trap maintenance program can be considerable. If 25% of the steam traps in a facility that has 100 steam traps can be repaired, then the annual savings (assuming $3,500 average savings per steam trap) is $87,500.

The table below provides estimates of leaking steam discharge rates for various size traps under different steam pressure conditions.

Leaking Steam Discharge Rate

Steam Loss (lb/hr)

 

Steam Pressure (psig)

Trap Orifice
Diameter (inches)

15

100

150

300

1/32

0.85

3.3

4.8

-

1/16

3.4

13.2

18.9

36.2

1/8

13.7

52.8

75.8

145

3/16

30.7

119

170

326

1/4

54.7

211

303

579

3/8

123

475

682

1,303

From the Boiler Efficiency Institute. Steam is discharging to atmospheric pressure.

Steam Trap Types

There are different types of steam traps for a variety of applications and requirements:

  • Float-and-thermostatic traps consist of a ball float and a thermostatic bellows element.
  • Fixed-orifice traps are self-regulating traps that continually discharge condensate.
  • Thermostatic traps have a main element that is a metallic, corrugated bellows filled with an alcohol mixture that has a boiling point lower than water. The bellows contracts when in contact with condensate and expands when steam is present.
  • Inverted bucket traps feature a bucket that raises and lowers as steam and condensate enter the trap body. These traps may be subject to freezing in low temperature climates if not insulated.
  • Thermodynamic traps feature a disk that rises and falls, depending on the difference in pressure between steam and condensate.

Testing Steam Traps

Steam traps should be tested to determine if they are functioning properly and not leaking, plugged, or failing in an open position to allow live steam to escape (blow by) into the condensate-return system. There are four basic ways to test steam traps: temperature, acoustic, visual, and electronic.

Plugged traps are cool while operating and leaking traps are hot. It is recommended that the temperature of a trap be taken with a non-contact, infrared thermometer.

In acoustic testing, an inspector understands the variances in the acoustic patterns of working or failed traps. As an example, float-and-thermostatic and fixed-orifice traps continually discharge condensate. For fully operating traps, the inspector hears a modulating, continuous flow. A blow-by condition changes this modulating flow to an intense, continuous rushing sound, indicating that the trap is not operating properly. If the trap has become plugged, there is no sound and the inspector can easily identify the problem.

Other types of traps have different acoustic patterns. The hold-discharge-hold pattern is typical of working thermostatic, inverted bucket, and thermodynamic traps that have intermittent open and closed cycles. For a failed trap, a blow-by condition is heard as a continuous rushing sound.

Electronic testing can involve the use of ultrasonic testers to better "hear" what is going on inside the trap. The procedure typically involves touching the trap on the downstream side with the instrument's contact probe and adjusting the sensitivity to better hear the flow.

Suggested Actions

Establish a program for the regular systematic inspection, testing, and repair of steam traps. Poorly operating steam traps can be quite costly. It is recommended that steam traps be tested at the following intervals to minimize energy losses through steam trap failures:

  • High Pressure (150 psig and above): weekly to monthly
  • Medium Pressure (30 to 150 psig): monthly to quarterly
  • Low Pressure (below 30 psig): annually
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