Managing Low Pressure Steam Systems For Efficient Heating

Managing Low Pressure Steam Systems For Efficient Heating

Steam heating systems, when properly maintained, provide efficient operation and excellent heating. Understanding how the systems operate and how the components function is important for proper maintenance.Steam heating systems generally fall into two categories—older one pipe systems and newer two pipe systems. Some of these are designed to operate under vacuum to speed up the flow of steam and provide even heating. Vacuum systems should be maintained as vacuum systems unless they are re-piped and zoned to provide even heating through the system.

We will limit discussion to the two pipe systems that have one pipe to deliver steam to the heating units and a second pipe to drain condensate. They use a steam trap to hold back steam until the latent heat is removed.

In this configuration, water is heated and converted to steam in the boiler. The steam leaves the boiler through the distribution pipes. Air in the pipes and heating units must be vented to allow the steam to enter so steam traps allow the air to discharge into the return line and to a vented condensate or boiler feed unit. As the steam gives up its heat, it condenses and the condensate drains through the steam trap to a condensate pump or boiler feed unit. The most common problem in these systems is failed steam traps.

The three types of traps generally used in steam heating systems include thermostaticfloat and thermostatic, and inverted bucket traps. A failed steam trap will allow steam to pass into the return line and to the condensate return units. The high-temperature condensate causes pump cavitation, mechanical seal failure, energy loss out the vent, and excessive boiler makeup water. A simple check on system operation is to monitor and record the condensate temperature at the condensate transfer units. A sudden increase in return temperature indicates that the steam traps require maintenance.

Thermostatic Trap

Thermostatic steam traps are the most common type found in two pipe steam heating systems. They are simple in construction and small in size and weight. They have adequate capacity at the low pressure differentials found in the average steam heating system. Thermostatic traps are the least expensive traps and are easily serviced.

A thermostatic trap is normally open to quickly discharge air and condensate (water) during a cold start up. They close in response to temperature as they get close to the saturation temperature of the steam. Most thermostatic traps close 10 to 30 deg F below the steam saturation temperature. In a low pressure steam heating system operating at 3 psig, the saturation temperature is 222 F. This means the trap will discharge condensate somewhere between 212 and 192 F. The return lines in steam heating systems are normally not insulated; this provides another 20 to 30 deg F subcooling before the condensate reaches a condensate return pump. The normal condensate temperature at the pump should be 160–180 F. This subcooling assures that there is no heat loss due to flash steam escaping from the receiver vent. It also allows a simple single stage centrifugal condensate pump to return the condensate.

Float and Thermostatic Traps

Float and thermostatic (F&T) traps have separate elements for venting air and draining condensate. The thermostatic element is located above the trap condensate level to allow free air venting during start up. The thermostatic element is similar to those in thermostatic traps. Air entering the trap body is discharged through the thermostat seat. The opening and closing of the thermostat seat depends on the temperature surrounding the thermostatic element.

Initially, condensate fills the trap body to a level slightly above the trap seat. It is sufficient to seal the valve seat opening, kept closed by the valve pin. As condensate continues to enter the trap body, the float will rise, causing the valve seat to be opened. Condensate then will be discharged through the trap outlet. The rise and fall of the float, due to changes in the water level within the trap body, results in a modulated discharge which is characteristic of F&T traps.

Inverted bucket traps

Bucket traps are so named because the operating mechanism inside the trap resembles an upside down open bucket. The bucket is fastened to the end of a lever, and the valve pin is fastened to the same lever near the fulcrum. A small bleed hole in the top of the bucket allows air to vent from the bucket.

The bucket trap opens when the bucket is filled with condensate and lifts to close the trap when it is filled with steam. Thebucket trap operation is open-close; it does not modulate.

Efficient operation

For low pressure steam systems to heat efficiently:

square.gif (108 bytes) Boilers should be set for operation at the minimal pressure that will provide the required heating. Condensate temperature and resulting flash loss relates to operating pressure.
square.gif (108 bytes) Most heat exchangers heating fluids to under 200 F should be sized for 2 psi steam operating pressure.
square.gif (108 bytes) Probe boiler controls should be cleaned or replaced annually.
square.gif (108 bytes) Float type boiler controls should be blown down frequently in accordance with the manufacturer’s recommendation to prevent the build up of dirt inside the float housing.
square.gif (108 bytes) Boiler blowdown should be performed according to the recommendations of the water treatment company.
square.gif (108 bytes) Use thermostatic traps where possible to take advantage of subcooling.
square.gif (108 bytes) Insulate steam lines to reduce heat loss in the distribution system. Don’t insulate the return line if it results in flash loss or pump cavitation. n Install condensate return units where condensate is being dumped. Monitor and record condensate temperatures weekly.
square.gif (108 bytes) Keep traps in good condition to prevent steam from passing into the return system. Test traps at least once a year on low pressure systems.
square.gif (108 bytes) Make sure the system vents properly—air trapped in the system acts as an insulator and results in poor heating.
square.gif (108 bytes) Avoid lifts in the gravity return lines that can cause poor venting and water hammer.

This article was originally published in the October 1998 issue of Maintenance Technology magazine.

Reprinted from TechTalk January 1999