ASHRAE Journal - September 2009 - 56

ASHRAE 19 and air rates), to maintain boiler pressure within an acceptable band. Safety valves are always installed on steam boilers to relieve excess steam pressure in case the combustion system should fail, and pressure starts to rise too high. The piping that connects the boiler to the system is especially important because it acts to separate liquid carryover from the steam flow. Most low pressure boilers do not have internal steam separating devices, so the steam header must be large enough, and properly designed to reduce steam velocity and permit liquid separation. The manufacturer’s installation drawings must be followed to get rated output without excessive carryover. 50 59–2009 YE A R S JOURNAL f i g . 1 Radiator vent valve. One-Pipe Systems These are called one-pipe systems because there is only one pipe connection at the radiator. There is always counter flow in this steam supply/ condensate return pipe and, therefore, it must be located at the bottom of the radiator. A vent is provided at the far end of the radiator (about one-half of the way up from the bottom) because the steam, which is less dense than air, fills the radiator from the top down, forcing the air out. Vents in the radiators and at the end of each main have four important jobs to perform. They must: • Open to vent air, allowing steam to fill the main or radiator. • Close to prevent steam loss and minimize the need for adding make-up water. Water added to the system brings in dissolved gasses and chemicals that cause boiler pitting, scaling and other problems. 56 ASHRAE Journal • Close to prevent loss of condensate if the radiator floods. • Open at the end of the heating cycle to break the vacuum caused by condensing steam and allow condensate drainage. If a partial vacuum was allowed in the radiator, condensate could be retained in the piping. Then, at the start of the next heating cycle, high velocity steam could accelerate these slugs of water through the pipe to cause damage and noise from violent water hammer. Thermostatic vents like those shown in Figure 1 have a small float that is partially filled with a mixture of alcohol and water. In the presence of air, the float’s weight keeps the vent open. As steam enters the vent body, the volatile mixture in the float begins to boil, expanding and causing the bottom of the float to bulge downward, driving the float pin into the seat to close the vent until the steam condenses. If condensate is retained in the vent, or if the radiator becomes flooded, the float rises to close the vent and prevent leakage. a s h r a e. o rg The vent top cap in Figure 1 has a number of different size holes that can be lined up with the vent seat. Radiators toward the end of the main use the larger holes, radiators close to the boiler use the smaller ones. As system pressure rises, the differential venting rates will allow all radiators to come up to operating temperature at about the same time, giving more even heating. The radiator supply valve is designed especially for one-pipe service and counter-flow of steam and condensate. It must be operated either wide open or tightly shut. If it was partly closed or if a standard globe valve was used by mistake, the crosssectional area available for steam flow would be reduced, causing steam velocity to increase. High velocity steam entering the radiator can cause condensate to back up, leading to problems in maintaining boiler level, system noise and reduced output from the radiator. Because the supply valve cannot be throttled, zoning in one-pipe sysSeptember 2009
ASHRAE Journal - September 2009

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