This code applies to steam boilers operating at pressures not greater than 15 psig and hot water heating boilers operating at pressures not greater than 160 psig and/or temperatures not greater than 205 deg F.

Steam Boilers

Valve capacity must be selected to prevent the boiler pressure from rising more than 5 psig above the MAWP.

Steam to Hot Water Supply:

Valve must be at least 1-inch diameter with set pressure not greater than MAWP of the tank.

Hot Water Boilers

Safety valve must be set to relieve at a pressure not greater than the MAWP of the boiler. If more than one safety valve is used, the secondary valve(s) may be set up to 6 psig above the MAWP for boilers with MAWPs up to and including 60 psig, and 5% for boilers with MAWPs greater than 60 psig. Capacity must be selected to prevent the pressure from rising more than 10% above the MAWP if one valve is used or 10% above the set pressure of the highest set valve if more than one valve is used.

Tanks/Heat Exchangers:

High Temperature Water-to-Water Heat Exchanger
Valve(s) must be set a pressure not greater than the MAWP and with sufficient capacity to prevent the pressure from increasing more than 10% above the MAWP.
High Temperature Water to Steam Heat Exchanger
Valve must be set at a pressure not greater than 15 psig and with sufficient capacity to prevent the pressure from rising more than 5 psig above the MAWP.

Set Pressure Tolerances:

Steam: For pressures of 15 psig, the tolerance shall be +/- 2 psig.
Hot Water: For pressures of 15 psig up to and including 60 psig, the tolerance shall be +/- 3 psig.
For pressures of 61 psig up to and including 160 psig, the tolerance shall be +/- 5%.

Single Valve Other Than Unfired Steam Boilers:

The single valve shall prevent the vessel pressure from rising more than 10% above the maximum allowable working pressure.

The valve shall be set to relives at a pressure not to exceed the maximum allowable working pressure of the vessel.

Multiple Valves on a Vessel Other Than Unfired Steam Boiler:

The aggregate capacity of multiple valves connected to any vessel or system of vessels for the release of liquid, air, steam, or other vapor shall be sufficient to relieve the maximum capacity that can be generated or supplied to the attached equipment without permitting a rise in vessel pressure to more than 16% above the maximum allowable working pressure.

One valve is to be set at or below the maximum allowable working pressure, the balance of the valves may be set at higher pressures up to, but may not exceed 105% of the maximum allowable working pressure.

Fire and/or External Heat Protection of Vessels Other Than Unfired Steam Boilers:

Supplemental valves for the protection from unexpected sources of external heat shall be capable of preventing the vessel pressure from rising more than 21% above the maximum allowable working pressure.

Multiple valves used to provide protection against excessive pressure caused by exposure to fire or other sources of external heat shall be set to operate at a pressure not in excess of 110% of the maximum allowable working pressure.

On a single valve installation to protect a vessel, and to provide fire/external heat protection, it shall not be set at a pressure above the maximum allowable working pressure.

Set Pressure Tolerances For the Above:

For pressures of 15 psig up to and including 70 psig, the tolerance shall be +/- 2 psig.
For pressures of 71 psig and above, the tolerance shall be +/- 3%.

Fire and/or External Heat Protection of Vessels Having No Permanent Supply Connection and Used for Non-Refrigerated Liquified Compressed Gases:

Valves shall be sized to prevent the pressure from rising more than 20% above the maximum allowable working pressure of the vessel.

The valve set pressure shall not exceed the maximum allowable working pressure of the vessel.

Set Pressure Tolerances For the Above:

The set pressure tolerance shall be -0 psig to +10%.

Download pdf

Accumulation: Accumulation is the pressure increase over the maximum allowable working pressure of the vessel during discharge through the pressure relief valve, expressed as a percentage of that pressure or actual pressure units.

Back Pressure: Back pressure is the pressure on the discharge side of a safety relief valve. This can either be constant or variable.

Blowdown: Blowdown is the difference between the set pressure and the reseating pressure of a pressure relief valve expressed as a percentage of the set pressure or actual pressure units.

Built-Up Back Pressure: Built-up back pressure is pressure which develops at the valve outlet as a result of flow, after the safety valve has been opened.

Chatter: Chatter is the abnormal, rapid reciprocating motion of the movable parts of a valve in which the disc contacts the seat.

Closing Pressure: Closing pressure is the point at which the valve re-closes. Closing pressure on a test stand may differ from the blowdown, which is the closing pressure under actual service conditions.

Cold Differential Test Pressure: Cold differential test pressure is the set pressure at which the valve is adjusted to open on the test stand. This pressure includes the corrections for back pressure and/or temperature service conditions.

Differential between operating and Set Pressures: Valves in process service will generally give best results if the operating pressure does not exceed 80% to 90% of the set pressure. However, on pump and compressor discharge lines, the differential required between the operating and set pressure may be greater because of pressure pulsations coming from a reciprocating piston. It is recommended that the valve be set as high above the operating pressure as possible.

Flutter: Flutter is the abnormal, rapid reciprocating motion of the movable part of a valve in which the disc does not contact the seat.

Lift: Lift is the actual travel of the disc away from the closed position when a valve is relieving.

Maximum Allowable Working Pressure: Maximum allowable working pressure is the maximum gauge pressure permissible in a vessel at a designated temperature. A vessel may not be operated above this pressure, or its equivalent, at any metal temperature other than that used in its design. Consequently, for that metal temperature, it is the highest pressure at which the primary safety relief valve is set to open.

Operating Pressure: The operating pressure is the gauge pressure to which the vessel is normally subjected in service.

Overpressure: Overpressure is a pressure increase over the set pressure of the primary relieving device. Overpressure is similar to accumulation when the relieving device is set at the maximum allowable working pressure of the vessel. Normally, overpressure is expressed as a percentage of set pressure.

Rated Capacity: Rated capacity is the percentage of measured flow at an authorized percent overpressure permitted by the applicable code. Rated capacity is generally expressed in pounds per hour for vapors, standard cubic feet per minute for gasses, and gallons per minute for liquids.

Relief Valve: A relief valve is an automatic pressure-relieving device, actuated by a static pressure upstream from the valve. This type of valve is used primarily for liquid service.

Safety Relief Valve: A safety relief valve is an automatic pressure-relieving device which may be used as either a safety ore relief valve depending upon application.

Safety Valve A safety valve is an automatic pressure-relieving device actuated by the static pressure upstream of the valve, characterized by rapid opening or pop action. This type of valve is used for steam, gas or vapor service.

Seat Tightness Pressure: Seat tightness pressure is the specified inlet static pressure at which a quantitative seat leakage test is performed in accordance with a standard procedure.

Set Pressure: Set pressure is the gauge pressure at the valve inlet, for which the safety relief valve has been adjusted to open under service conditions. In liquid service, set pressure is determined by the inlet pressure at which the valve starts to discharge. In gas or vapor service, the set pressure is determined by the inlet pressure at which the valve pops.

Simmer: Simmer is characterized by the audible passage of a gas or vapor across the seating surfaces just prior to “pop”. The difference between this “start to open pressure” and the set pressure is simmer, and is generally expressed as a percentage of set pressure.

Valve Trim: Valve trim includes the nozzle and disc.

Safety Relief Valve Pointers

1. ASME Codes require that valves for air, steam and water service over 140 deg F. have test levers.

2. Steam safety valves may be used for air service, but not vice versa. Liquid valves should only be used on liquid service.

3. Safety relief valves should be installed vertically with the drain holes open or piped to a convenient location.

4. The inlet to and outlet from a safety relief valve must be at least as large as the inlet and outlet connections of the pressure relief valve.

5. The lever of a safety valve is not to be used as a carry handle.

6. Hydro-testing of a boiler should never be performed with the safety valves installed.

Maintenance

Per the National Board

Inspection areas of concern include:

A. Safety considerations
B. Device data
C. Condition of the device
D. Condition of the installation
E. Testing and operational inspection.

Per the National Board

Check for evidence that the valve or device is leaking or not sealing properly. Seals for adjustments should be intact and show no evidence of tampering. Connecting bolting should be tight and all bolts intact. The valves should be examined for deposits or material build up. Evidence of rust or corrosion should be checked. Check for damaged or misapplied parts. If a drain hole is visible, ensure it is not clogged with debris or deposits. Check for test gags left in place after pressure testing of the unit. Bellow valves shall be checked to ensure the bonnet vent is open or piped to a safe location. The vent shall not be plugged since this will cause the valve set pressure to be high if the bellows develops a leak. Leakage noted from the vent indicates the bellows is damaged and will no longer protect the valve from the effects of back pressure.

Per the National Board

Inspect the inlet piping and ensure that the pipe size is not smaller than the device inlet size. Inspect the discharge piping and ensure that the discharge pipe size in not smaller than the device outlet size. Check that the valve drain piping is open. Check drainage of discharge piping. Check that the inlet and discharge piping are not binding or placing excessive stress on the valve body which can lead to distortion of the valve body and leakage or malfunction. Check that there are no intervening isolation valves between the pressure source and the valve inlet or between the valve outlet and its point of discharge (unless otherwise permitted).

Recommended Inspection and Test Frequencies:

Per the National Board

Power Boilers

A. Pressure less that 400 psig: Manually check every 6 months; pressure test annually to verify nameplate set pressure or as determined by operating experience as verified by testing history.

B. Pressure greater than 400 psig: Pressure test to verify nameplate set pressure every three years or as determined by operating experience as verified by testing history. Pressure tests should be performed prior to bringing the boiler down for planned internal inspection so needed repairs or adjustments can be made while the boiler is down.

High Temperature Hot Water Boilers

Pressure test annually to verify nameplate set pressure or as determined by operating experience as verified by testing history. For safety reasons, removal and testing on a steam test bench is recommended. Such testing will avoid damaging the safety valve discharge of a steam water mixture, which could occur if the valve is tested in place.

Low Pressure Steam Heating Boilers

Manual check quarterly; pressure test annually prior to steam heating season to verify nameplate set pressure.

Hot Water Heating Boilers

Manual check quarterly; pressure test annually prior to steam heating season to verify name.

Water Heaters

Manual check every two months. Due to the relatively low cost of safety valves for this services, it is recommended that a defective valve be replaced with a new valve if a repair or resetting is indicated.

Pressure Vessels and Piping

Frequency of test and inspection of pressure relief devices for pressure vessel and piping service is greatly dependent on the nature of the contents and operation of the system and only general recommendations can be given. Inspection frequency should be based on previous inspection history. If valves are found to be defective or damaged by system contents during inspection, intervals should be shortened until acceptable inspection results are obtained. Where test records and/or inspection history are not available, the following inspection and test frequencies are suggested.

How do I convert boiler horsepower to pounds of steam?

1 boiler horsepower is equal to 34.5 pounds of steam per hour

Example: (300 BHP) * (34.5 lbs of steam per hour) = 10350 lbs/hr

How do I convert boiler horsepower to BTUs per hour?

1 boiler horsepower is equal to 33472 BTU per hour

Example: (100 BHP) * (33472 BTU per hour) = 3347200 BTU per hour

How do I change cfm to scfm?

1 boiler horsepower is equal to 34.5 pounds of steam per hour

scfm = (cfm) * (14.7+P) * (520)
14.7 460+T

P = gauge pressure of gas or vapor in psig
T = temperature of gas or vapor in deg F

Example:
scfm = (200 cfm) * (14.7 + 25) * (520)
14.7 460+120

scfm = 485 scfm