Sizing the safety valve

[mtnman]

OK after several design changes and size changes the final size for my boiler has been determined. 18” diameter, 20” long with 61 1”OD tubes, this is just the pressure vessel not including the firebox and ash pit. My question is: What size safety valve will this boiler need? I’ve been reading about optimum size safeties and have learned that you don’t want to be too big or too small, but I’ve not found a formula to determine the correct size. What say you?

[stevey_frac]

I'm not the expert on this. Actually i'm not an expert at all! But I believe the rule of thumb is that the safety should be able to vent the total production capacity of your boiler + 50%. I.E. If your boiler can throw out 200 lbs steam/hr, the safety needs to be able to vent 300 lbs/steam per hour.

[artemis]

I'm not the expert on this. Actually i'm not an expert at all! But I believe the rule of thumb is that the safety should be able to vent the total production capacity of your boiler + 50%. I.E. If your boiler can throw out 200 lbs steam/hr, the safety needs to be able to vent 300 lbs/steam per hour.

NO. Having too large a safety valve is almost as bad as having none at all. Either the valve will not lift properly causing "wire drawing" on the seat or it will rapidly "open-close, open-close" damaging the seat.

The main purpose of the safety valve is to prevent the pressure in the system exceeding Maximum Allowable Working Pressure (MAWP). Above this pressure, no one can guarantee the system safety - and especially for a steam system with very hot gas with huge amount of latent heat, the consequences can be dramatic.

The size of the safety valve depends primarily on the maximum boiler output and the working operating pressure of the system - this is important as a lower prressure will create a greater VOLUME of steam per POUND of WEIGHT. The safety valve shall, as a minimum, have the evacuation capacity of all the vapor the boiler can produce running at full power at the WORKING OPERATING pressure.

1. Select the valve of the proper pressure rating and rated pound discharge capacity to accommodate the particular boiler. The minimum required relieving capacity for valves on ASME Section I ‘S’ Code boilers shall not be less than the maximum designed steaming capacity as determined by the manufacturer. If your don't know this then a close enough approximation may be made by multiplying the boiler heating surface in square feet by 5 for a firetube boiler, and by 10 for a water tube boiler.

2. The safety valve set pressure must never be in excess of the MAWP pressure stamped on the boiler. For high pressure boilers (over 15psi), the maximum boiler working operating pressure must always be at least 10% below (but not less than 7psi below) the safety valve set pressure to prevent leakage. Thus if you have a boiler with a MAWP of 100psi, then your maximum working operating pressure should be no more than 90psi. Failure to maintain this operating gap can result in an accumulation of deposits on the seating surface, which may impede proper operation of the safety valve.

Install the safety valve in a vertical upright position at the opening provided and recommended by the manufacturer as close to the boiler as conveniently possible. No shutoff valve or other connections or restrictions are permissible between the boiler and the safety valve. The safety valve piping and opening from the boiler cannot be less than the inlet size of the valve, but the valve inlet may be smaller than the boiler outlet providing the safety valve has the required relieving capacity.

The safety valve is a precision made instrument and should be handled with care. Before installing be sure that:
(a) All upstream pipes and connections have been blown clean.
(b) Pipe compound is used on external threads only.
(c) Inlet port of valve is free of any foreign material.
(d) Use wrench on valve inlet hex only and avoid over tightening. Do not use a pipe wrench.

Do not cap or plug the drain hole in the side of the valve body.

TO AVOID WATER DAMAGE OR SCALDING DUE TO VALVE OPERATION, A DISCHARGE PIPE MUST BE CONNECTED TO THE VALVE OUTLET AND RUN FULL SIZE TO A SAFE PLACE OF DISPOSAL. IF A DISCHARGE PIPE IS NOT INSTALLED CORRECTLY, PERSONAL INJURY AND PROPERTY DAMAGE COULD RESULT. The discharge line must be piped independent of all other piping, as short and straight as possible, without any intervening valve of any description and terminate freely to atmosphere. Never restrict or block a safety valve outlet. Install piping with sufficient flexibility to allow for free expansion and properly support it so there is no strain on the safety valve body. Pipe to a safe point of discharge to prevent any possibility of personal injury. If piped upwardly, a drain line should be provided at the low point to keep this line drained. Secure the piping so it cannot move to cause personal injury when safety valve discharges. Use schedule 40 pipe only for discharge line (do not use schedule 80, extra strong or double extra strong discharge pipe or connections).

It is recommended that the safety valve be tested periodically. Test also at the beginning and end of any extended non-service period. A loud noise and high velocity steam will discharge freely from discharge port and through drain hole provided in the side of valve body. The test lever is designed to be activated only when 75% or more of the popping pressure is reached, otherwise distortion could result. The valve should be tested at or near maximum operating pressure by holding the test lever fully open for approximately 5 seconds to flush the valve seat free of any debris or sediment then permit valve to snap shut. If the valve leaks, raise and lower the handle slowly and be sure the valve properly reseats. If valve continues to leak, shut down the boiler and replace valve before placing boiler back in service.

Most manufacturers or retailers of steam safety valves can set you up with what you need.

Or if you need more, Google "sizing steam safety valves".

[PeteThePen1]

Thanks for that Ron

You have given us a nice summary of the stuff we should all know, but (speaking for myself) probably only half knew. Three cheers for the Forum - how did the early steam revivalists get by without such a useful source of information?

Regards

Pete

[artemis]

Thanks for that Ron

You have given us a nice summary of the stuff we should all know, but (speaking for myself) probably only half knew. Three cheers for the Forum - how did the early steam revivalists get by without such a useful source of information?

Regards

Pete

Darwin's "survival of the fittest" in action. Those who did proper work survived to pass on their teachings. Those who didn't became characters in an early steam best seller titled "Limbs Along the Levee". <grin>

[fredrosse]

ASME Guide for determining steaming capacity of power boilers
Pounds of steam Per Hour , Oil or Gas Fired, for each square foot of heating surface, PPH/ft2

Firetube boilers,
Furnace surfaces, 14 PPH/ft2
Tube surfaces, 8 PPH/ft2

Watertube Boilers
Furnace Waterwall surfaces, 16 PPH/ft2
Tube surfaces, 10 PPH/ft2

These are only guide numbers, the ASME Code directs that the boiler manufacturer is to determine the maximum steaming capacity, and size safety valves to pass this flow at the Maximum Allowable Working Pressure (MAWP), plus some small margin.

Note that higher steaming rates are often available with a forced fire, I have seen as much as 50 PPH/ft2 in the furnace areas of large utility boilers burning pulverized coal.

[artemis]

The figures given reflect the characteristics of large, commercial boilers. In our "little" boats and "minature" boilers those can be very skewed as witness the sometimes improbably high - 50 pounds/horsepower/hour - steam consumption of small engines.

[stevey_frac]

Apparently the information I read on safety valve sizing was dead wrong!

That's disquieting.

Note to self: Ask on these forums before doing *anything*.

[fredrosse]

The numbers quoted for ASME are applicable to their "Miniature Power Boiler" classification. The ASME defines any steam boiler operating above 15 PSIG (about one Barg) as a "Power Boiler".

The ASME miniature classification limits the overall size of the basic boiler to 5 cubic feet of volume, 16 inch maximum shell size, and 20 square feet heating surface, and 100 PSIG maximum allowable working pressure. This is often very much in line with steam launch boilers.

The boiler I just had made is an ASME miniature class boiler, 16 inch dia shell, 20 square feet heating surface. It could have been classed as a 250 PSIG power boiler without the miniature classification, but I did not want to put the fabrication shop through extra review cycles, and my walking beam engine is happy with 75 PSIG steam.

[artemis]

The numbers quoted for ASME are applicable to their "Miniature Power Boiler" classification. The ASME defines any steam boiler operating above 15 PSIG (about one Barg) as a "Power Boiler".

The ASME miniature classification limits the overall size of the basic boiler to 5 cubic feet of volume, 16 inch maximum shell size, and 20 square feet heating surface, and 100 PSIG maximum allowable working pressure. This is often very much in line with steam launch boilers.

The boiler I just had made is an ASME miniature class boiler, 16 inch dia shell, 20 square feet heating surface. It could have been classed as a 250 PSIG power boiler without the miniature classification, but I did not want to put the fabrication shop through extra review cycles, and my walking beam engine is happy with 75 PSIG steam.

I do somewhat question the values of the ASME in FTs. They might be OK for HFTs but the heat transfer in VFTs is more questionable as, after a distance into the tube, the turbulence of the heat gases in the tube turns into a laminar flow with resulting loss of heat transfer. I'm planning on using a VFT in Pond Skimmer and trying to keep all of this as compact as possible and still provide enough steam for a small 1.3HP engine at 350 rpm. Assuming a woefully inefficient engine (it's 2" x 1.9"), I'll need about 45# of steam an hour at 125 psig or 6 sq.ft. of heating surface - no waterwall firebox as I'm using propane and "blue flame" is not a good radiant heat source. This also is where calculating the boundary between turbulent and laminar flow (Reynold's number and all that - gah!) comes in. Any thoughts you have along that line are welcome.