Boiler safety valves are critical in preventing over-pressurisation and comprise the final boiler over-pressurisation protection device.
A fire-tube boiler can be fitted with one or more safety valves on the top of its shell, with each set to open when the boiler reaches its design pressure. No isolation valves or restrictions should be integrated between the safety valve(s) and boiler. If the valves are not installed directly onto the boiler shell, the pipework connecting the valves to the boiler must be kept clear of blockages and water, and this must be confirmed by periodic testing.
Firetube Boiler Safety Valves
Once a safety valve opens, steam is discharged via the exhaust pipe. Exhaust pipes must be designed to encounter as few bends as possible, be as short as possible, to have no reduction in pipe section (no internal pipe diameter reduction), and should lead to a safe point of discharge (typically outside the boiler house).
Water must be drained from the safety valve or exhaust pipework via a drainpipe. Drainpipes may be connected to holes drilled into the lowest section of the exhaust pipework, or, directly to drain holes in the safety valve body; these drains are not to be confused with the blowdown ring locking bolt, if one is fitted.
Where two safety valves are fitted, it is common that one is set just below the boiler’s design pressure. It is vital that each safety valve permits the full flow of steam produced when the boiler is operating at maximum capacity i.e. when the boiler is producing the maximum amount of steam it can possibly produce. If safety valves are sized correctly, a boiler can be firing at full capacity without the steam pressure exceeding design limits (because the safety valve(s) relieves pressure at a faster rate than it is accumulated).
Safety Valve Types
There are various types of safety valve, including high lift and improved high lift valves, which use the force of escaping steam to open a winged valve plug to achieve greater steam flow rates. In addition to this, some valves integrate a piston at the bottom of the spring chamber. The piston has a larger surface area than the valve plug, which leads to the valve opening with a definitive ‘pop’ sound.
Boiler Safety Relief Valve (SRV) Cross-Section
Some boiler safety valves include a blowdown ring. The blowdown ring can raise or lower the valve seat ring and is used to control the amount of blowdown through the valve. This ring is locked by a bolt that protrudes through the valve and into the adjusting ring segments.
Boiler safety valves should be fitted with an easing gear (looks like a handle), used, when necessary, to rapidly release boiler pressure. Easing gears can also be used for testing a safety valve, ensuring the spindle has freedom of movement and that the valve operating mechanism functions as intended. Easing gear testing is often not conducted due to operators having difficulty with the valves resealing, but this is generally only the case with valves that are not tested often enough. Actuating the easing gear several times is often all it takes to dislodge debris from the sealing area and allow the valve to seal again. For safe operation, the easing gear handle is usually connected via steel cables to an area neighbouring the boiler.
Safety and Relief Valve Maintenance
Like pressure gauges, all safety valves should be stripped, inspected, and calibrated, at least once a year; maintenance usually occurs during statutory inspections. Calibration of each valve should be conducted by a competent person, and any valve adjustment (including the blowdown ring) should be approved and sealed by the authorised inspector. After testing and calibration, all valves should be correctly marked, suitable certificates issued, and accurate records maintained.
An accumulation test can be conducted to ensure a safety valve can relieve over-pressure steam when the boiler burner is operating at maximum capacity. Accumulation testing of safety valves must be repeated after any alterations are made to the boiler e.g. replacement of a safety valve, fuel change, or changes to the control system. If, during an accumulation test, boiler pressure rises by more than 10% of its design pressure, the test must be aborted. Before the boiler is re-tested, amendments must be made to either the safety valve relieving capacity, the safety valve exhaust pipework, or the boiler’s steaming capacity, to ensure the 10% limit is never exceeded.
What is the difference between a relief valve and a safety valve?
Relief and safety valves prevent equipment damage by relieving over-pressurisation of fluid systems. The main difference between a relief valve and a safety valve is the extent of opening at the set-point pressure.
A relief valve gradually opens as the inlet pressure increases above the set-point. A relief valve opens only as necessary to relieve the over-pressure condition. Relief valves are typically used for liquid systems.
A safety valve rapidly ‘pops’ fully open as soon as the pressure setting is reached and will stay fully open until the pressure drops below the reset pressure. The reset pressure is lower than the actuating set-point pressure. The difference between the actuating pressure set-point, and the pressure at which the safety valve resets, is called blowdown. Safety valves are typically used for gas or vapour systems.
Safety Relief Valve (SRV)
A safety relief valve may open fully, or proportionally, once the pressure setting is reached. SRVs may be used for any fluid system (gas, liquid, or vapour).
Other Relief Device Types
Other common relief devices include the rupture disk and temperature and pressure relief valve (referred to as a TPR or TPRV); see our water heater article to learn more about TPR valves and how they work.