Common Venting Boiler Safety Relief Valves and Natural Gas Valves
As is the case with most projects, the mechanical rooms are too small to allow for a simple installation of the required equipment, the chases are too small for the items that must pass through them, etc., etc. When managing construction projects, many of you probably feel as though you spend the majority of your time working to find creative ways to effectively minimize labor overages, while at the same time squeezing 10 pounds of beans into a 5 pound bag….on an already tight labor budget. While we cannot magically increase the size of the mechanical room for you or clone your top field personnel with this article, the following information will provide you with some options that will simplify your next boiler project.
I believe that on almost every boiler project that I have been involved in over the past few decades, there always seems to be confusion regarding the venting requirements for safety relief valves and gas regulator common venting. Below, I have referenced the codes that apply to these topics, in addition to some examples.
SPS 341 adopts CSD-1 2009 & B31.1 2010. SPS 341.05 has a PETITION FOR VARIEANCE which reads:
The department shall consider and may grant a variance to a provision of this chapter in accordance with ch. SPS 303. The petition for variance shall include, where applicable, a position statement from the fire department having jurisdiction, and from any first class city having jurisdiction to enforce this chapter.
CSD-1 has authority or the gas venting of the boilers and explains the following for combining vents. CF-190 paragraph (a):
Where there is more than one gas pressure regulator at a location, each gas pressure regulator shall have a separate vent to the outdoors at a safe point of discharge as determined by the authority having jurisdiction. If approved by the authority having jurisdiction, the vent lines may be manifolded in accordance with accepted engineering practices [see (b) and (c) below] to minimize backpressure in the event of diaphragm failure. (b) Atmospheric vent lines, when manifolded shall be connected to a common atmospheric vent line having a cross sectional area not less than the area of the largest vent line plus 50% of the area of the additional vent lines. (c) Atmospheric vent lines shall not be connected to any common manifolded gas vent, bleed or relief lines.
B31.1 has authority on the relief venting of Reliving Devices (SRV’s). 122.6.2 paragraph (C) reads:
(C) It is recommended that individual discharge lines be used, but if two or more reliefs are combined, the discharge piping shall be designed with sufficient flow area to prevent blowout of steam or other fluids. Sectional areas of a discharge pipe shall not be less than the full area of the valve outlets discharging thereinto and the discharge pipe shall be as short and straight as possible and so arranged as to avoid undue stresses on the valve or valves.
After reviewing the codes detailed above, you can see that contrary to popular belief, it is possible to incorporate a common vent manifold for boiler safety relief valves, and also for gas valves. Below are some examples.
Gas Valve Common Vent Manifold:
Manifold gas vent lines common (1 ¼” each boiler x (4) Units (1.84” + .92” +.92” + .92” = 4.6”) therefore, a 2 ½” manifold would be recommended (cross sectional area of 4.9”)
Safety Relief Valve Common Vent Manifold:
Manifolded relief valves (I don’t interpret this code to limit only the boilers to this vent line but would include DA and pressure reducing stations)
Boilers (2) 2 ½” relief valves per boiler (4.9” cross sectional area each) x (4) Boilers (39.2” total) therefore, an 8” manifold would be recommended (50.2” cross sectional area).
The same methodology can be used when venting pressure reducing stations and deaerators per the referenced code with proper Department approval. Remember, the purpose of a Drip Pan Elbow is to provide a suitable unrestricted, self-draining outlet in addition to isolating Safety Relief Valves (SRV’s) from discharge piping stresses. The Safety Relief Valve discharge size increases to isolate the drip pan elbow from backpressure. Steam won’t escape from the drip pan elbow if the vent line is sized appropriately.
If you run into a situation where common venting is required for safety relief valve(s), gas regulators, or pressure reducing stations, please feel free to contact Fluid Handling, Inc. in Menomonee Falls, WI and we will work with you to determine the appropriate solution.
Fluid Handling Inc.