Proper installation can avoid safety valve problems
By David C. Burgeson, P.gif.
A helpful reminder for designers and installers
Despite their importance, safety valves are often installed in a manner that almost guarantees problems. A designer must keep a few points in mind to avoid this.
A designer must provide enough clearance between the vent stack and the exhaust pipe or the drip pan to allow for thermal expansion. A safety valve can move as much as an inch or more when the pressure vessel or pipe to which it is attached expands as it heats. Without enough clearance the exhaust pipe or the drip pan can come in contact with the vent stack, which is usually fastened to a structure. This puts the safety valve in a bind and can distort the valve body, causing erratic operation. The same problem can occur if the drain pipes are not flexible enough to allow the safety valve to move without binding.
The reactive force of the steam coming out the exhaust pipe can also cause problems. When a safety valve blows there is a downward force on the elbow. This downward force can create a moment on the valve body which can cause erratic operation. To limit the moment, safety valve installers should make dimension “A” (see figure) as short as possible. Also addressing the stress on the valve body, the American Society of Mechanical Engineers (ASME) boiler code states “when a discharge pipe is used, the cross sectional area shall not be less than the full area of the valve outlet discharging thereinto. It shall be as short and straight as possible and so arranged as to avoid undue stresses on the valve.”
The ASME boiler code calls for safety valve drains to be open gravity drains. Many designers ignore this and tie the drains into other piping systems such as building floor or roof drains. This can allow water to collect in the safety valve body, rusting the internals so badly the valve becomes inoperable. Boiler solids entering the valve can also cause the valve to become inoperable if safety valve drains are tied into boiler blowdown lines. Furthermore, this arrangement can create a safety hazard, as boiler blow down water can spray out around the safety valve drip pans.
Improper installation, such as incorrect adjustment of the blowdown, flow restriction between the pressure vessel and the safety valve, or back pressure from an exhaust system that has too small a diameter, is too long, or has too many bends, can cause valve chatter.
A safety valve lifts when the pressure in the vessel reaches a certain point, stays open until the pressure drops by a few percent, then closes. The steam flow`s thrust on the valve disc keeps the valve open during the blowdown period. The thrust is provided by a change of direction of the steam flowing through the valve and/or the steam pressure acting on a larger area of the disc than when the valve is closed. If the flow is restricted or the back pressure is too large, the flow will be small and the thrust may not be adequate to keep the valve open. The valve will immediately lift again, as it hasn`t blown long enough to relieve pressure in the vessel. This scenario can occur over and over in rapid succession, causing chatter. The resultant hammering of the valve disc against the seat can destroy both disc and seat. Furthermore, if the safety valve is bolted to the pressure vessel and a flexible gasket is used, the vibration can cause the nuts to back off the studs that hold the valve in place.
Flow restrictions must be limited between the pressure vessel and the safety valve. The ASME boiler code says the intervening piping or fitting between the boiler and the safety valve (dimension “B”) shall “not be longer than the corresponding tee fitting of the same diameter and pressure … The opening or connection between the boiler and the safety valve shall have at least the area of the valve inlet.” z
David C. Burgeson, P.gif., of Burgeson Boiler Consultants Inc., has nearly 40 years` experience working in various capacities in fossil-fired power plants. His main expertise is boilers. For the past 22 years he has been a consultant, and prior to that he spent six years as chief engineer at Austin Utilities. He holds a bachelor`s degree in mechanical engineering from the University of Minnesota
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