Problems Making Safety Valve Operate

[paulge]

At long last have made 3" vert boiler with grateful thanks to SandyC for his inspiring design. Have tested at home to 100+psi.
Having trouble with safety valve. Have made open ball and enclosed. Both open at about right pressure (40psi using compressor). Neither will shut until about 20psi. What is the secret to getting them to close just below opening pressure? Had a look at Tony Bird construction pictures - very impressive. What am I doing wrong?

 

[Tin Falcon]

probably need to adjust or change the spring.
I purchased a commercial pressure relief valve it came with three springs. adjusting spring tension with the knurled knob fine tunes it.


ANd if it opens at the right pressure you may be doing nothing wrong. it is a pressure relief valve that is designed to open at a prescribed pressure not a pressure regulator.
Tin

 

[paulge]

Tin, thank you for advice. Will try changing spring tension.
paul

 

[Cogsy]

I second what Tin said, if it is opening at the correct pressure then it is doing it's job. I imagine the out rushing material (air/steam/whatever) keeps it open until the spring can overpower the outward flow and close again. I assume that if you change spring pressure you will change opening pressure.

I don't have any steam experience but in dealing with air tools/compressors, the safety valve is normally a fail-safe only and is not supposed to operate unless something has gone wrong. It's definitely not used as a pressure regulator.

 

[paulge]

Hi all, have tried various strength springs and finished with one that opens at about 42psi and closes at 30psi.
I take your comments and realize I was barking up the wrong tree.
Thank you for your support.

 

[Jyman]

Cogsy, is correct in what he's saying, in that the surface area of the valve increases when it opens requiring less pressure to keep the valve open.

I can help you out here with the safety valve, one way to decrease the range of your valve would be to decrease the difference between the closed and open area of the valve. The closer the two are to each other the less pressure drop over the area will be needed to close the valve after it opens.

Your not barking up the wrong tree with this question, because you never know if someone will have a answer to it. Or you might just get someone like me who's area of work is steam boilers.


Sent from my iPhone using Model Engines

 

[Charles Lamont]

The ball seating is important. The hole should be about 70% of the diameter of the ball, ideally reamed, and it should have a flat face cut with a slot drill or D-bit to a sharp, square edge.

Put a hard bearing ball of the same diameter on the carefully cleaned seat and with a bit of rod that slides in the body as a punch, give it one sharp tap with a small hammer to make a narrow seating.

Ideally the spring should have close-coiled ground ends to ensure the line of thrust is true. The pin should not need a deep cup for the ball.

By making a restriction in the diameter of the hole above the seating, up to about half way up the seated ball, and say 10-15% larger in diameter than the ball, you are making a 'pop' valve. The constriction means that it will open wider, but will snap shut more quickly.

 

[kf2qd]

Typically a safety valve will close at a much lower pressure than what is opened. This is for safety. If it opens then something is wrong and it is there to allow the pressure to drop to a safe point before it seats again. If this valve is opening often then your boiler heat is poorly controlled.

 

[Charles Lamont]

Typically a safety valve will close at a much lower pressure than what is opened. This is for safety.

No. The safety valve's function is to release excess steam, ideally without allowing the pressure to drop very far. That is why the pressure the safety valve is set to lift at is called the 'working pressure'. Closing at 'a much lower pressure' is exactly the wasteful behaviour the OP wants to avoid.

 

[paulge]

Hi All, thank you Charles for your thoughts. I have built several versions using your guide lines over the last week. Still not any nearer but have learned a lot about the variables involved. I had thought that my springs would obey Hooke's law ie be linear but perhaps because we only use a very small range of compression/extension the hysteresis effect dominates and this is what causes the large differential in opening and then closing pressures. I take on-board others comments that essentially it is a Safety Valve and therefore if it reliably opens at the required pressure then it is doing it's job.

 

[Charles Lamont]

Is the pin an easy (loose) fit in the nut at the top, and have you polished the shank of the pin?

 

[paulge]

Hi all, I have managed to control the safety valve and have completed my project thus far. Boiler tested, steam test tomorrow. Have attached a couple of pics. Thanks for all help. Paul

 

[RichD]

Nice job Paul!
Rich

 

[Gedeon Spilett]

the hystesis of these valves is difficult to circumvent. but a safety valve is a safety device, which must operate in the event of danger, and not to regulate the pressure, too much steam waste!
You have to install a pressure control valve for the gas burner for controlling the heating as a function of the boiler pressure...
nice launch you have made.

 

[paulge]

Thank you and merci. It has been a long saga but all ok now. Passed the steam test at Cheddar Steam Club so good to go.