Steam safety valve - narrow operating margins 1

[SFuller]

Our site has recently increased the set pressure of the relief devices on our steam boilers from 7.9 barg to 8.4 barg. This was done to allow the hardwired high pressure trip setting on the boilers to be increased, preventing the boilers from nuisance trips due to sudden drops in steam demand. Operating steam pressure is 7.5 barg on site.
I am looking at the requirements to fit pressure relief devices to steam receivers on site. Local legal requirements (Ireland) state that if the MAWP (or design pressure) of a steam receiver is less than the maximum permissible working pressure of the boiler a relief device must be fitted to the receiver.
Historically our receivers were specified with a design pressure of 8 barg to avoid the need to relief devices. Now they will have to be fitted.
My question is: As the receivers are AMSE vessels and a 10% accumulation is allowed by code, the relieving pressure of the valve will be 8.8 barg – this cannot be achieved by the boiler! In order to prevent the valve from chattering I can put the set pressure of the valve to 7.64 barg so that it reaches 10% over pressure at 8.4 barg and will relieve properly. On the other hand this is very close to the operating steam pressure so there is a good chance of the valve leaking. Does anybody have any ideas on what to do or know of any guidance in the codes?

sfuller

 

[Montemayor]

sfuller:

I understand your explanation to say that you will set the relief valves on the steam receivers ( design pressure = 8 barg) at 8.8 barg. You can't do this, if I am interpreting your intentions correctly. The relief set pressure on the steam receivers must be: 8 barg (the MAWP or "design pressure") - at least according to ASME. I don't know how the Irish code specifies the set pressure, but I assume they do the same. Accumulation is the amount of pressure over-and-above the PSV's set pressure that "accumulates" during the relief action.

The PSVs should start to relieve at 8 barg - not at 8.8 barg. I think we both understand that; I just want to make doubly sure that we interpret what the set value is for the PSV. Your boiler can attain 8.4 barg - according to your latest modification. That means that it is possible for your steam receivers to also reach that level. If they do, they are operating over their design pressure and they must be relieved as soon as that value is reached - which is the 8.0 barg value. I don't believe you have a case for "chattering". Chattering occurs when the PSV is grossly oversized; this is not mentioned by you.

 

[Latexman]

Accumulation = Vessel pressure above the MAWP, expressed as a percentage of MAWP (100 x [sizing pressure - MAWP]/MAWP).

Overpressure = the pressure difference between sizing (relieving) and set pressures, expressed as a percentage of set pressure (100 x [sizing pressure - set pressure]/set pressure).

Accumulation = Overpressure if Set Pressure = MAWP

Good luck,
Latexman

 

[SFuller]

Montmeyor,
I should have explained the problem more clearly. As you stated, the set pressure of the relief valve on the steam receiver will have to be equal to the MAWP (or the design pressure if the MAWP is not know) of the receiver. In this case the set pressure of the valve will be 8.0 barg. What I am questioning is that according to most relief valve catalogues that I have looked at, a valve will need to reach 10% overpressure (8.8 barg in this case) before it will achieve it's full rated flow. This cannot happen as here the boilers will only go to 8.4 barg.
If a valve is not at 10% over pressure, can I rely on it to relive the required quantity of steam properly?
Thanks for your help.
SFuller

 

[EGT01]

SFuller,

In addition to the point the Montemayor has made about boiler relief device set pressure versus receiver MAWP, if your boiler relief devices are set at 8.4 barg and they are designed per ASME Section I, they could experience 6% overpressure as allowed by the code.
8.4 * 1.06 = 8.9 barg

You don't give any details about your hardwired trip and I'm not experienced in boiler operations, so I don't know what value the trip would have for limiting the pressure in regards to overpressure protection of any of the affected equipment.

It would seem that the relief devices at the boiler were actually providing the required relief protection of the steam receivers when the boiler relief devices were set at 7.9 barg. It almost seems counter-productive to increase the set point of the boiler relief devices only to have to install new relief devices on your steam receivers, don't suppose there are any other options to address the nuisance trips.

Now if your steam receivers do fall under ASME Section VIII, you might be able to follow the rules for multiple relief devices. By installing a relief valve on your receiver set at MAWP (8 barg) and sized for some nominal rate, you may could claim that the relief device on your boiler set at 8.4 barg is part of your receiver protection.
8.4/8.0 = 105% of MAWP is allowed by Section VIII for a secondary relief device.

Considering both relief devices to be part of the receiver overpressure protection, you would be allowed up to 16% overpressure at the receiver (8.0*1.16 = 9.28 barg) since there are multiple relief devices and the relief device at the boiler is essentially still providing the protection.

As additional comments, are we to understand that your initial conditions were such that operating steam pressure was 7.5 barg with your relief devices set at 7.9 barg? That seems like a very close operating margin
7.5/7.9 ~ 95%
What type of relief devices do you have at the boiler?
7.5/8.4 ~ 90% is pushing the limit for a conventional type relief valve.

Relief valves certified to ASME Section VIII must achieve full rated capacity at no more than 10% overpressure. Relief valves certified to ASME Section I must achieve full rated capacity at no more than 3% overpressure. Either way, you will always select a relief valve with greater capacity than you need. As a rule of thumb, for valves in vapor service, as long as the required relief rate is no less than 25% of the valve capacity, the valve would be acceptably oversized and you shouldn't have to worry about chatter.