6 replies to this topic

[WilliamN]

I have a conventional pressure  relief valve on a steam line. The steam pressure is 650 psig while the PSV set pressure is 700 psig. The valve has been working fine for few years. But it does not close properly after it popped up first time in recent time .eg, steam comes out from normal operation. I thought the set pressure might be too close to the operating pressure. But I don't have control over the steam pressure, so I can not lower it. The set pressure of 700 psig is needed to protect the piping, so I can not raise it.

 

Appreciate any comments from  specialists from the forum.

[fallah]

William,

 

It might the cause of not closing the PSV after pop up to be low difference between set pressure and operating pressure...

 

Checking the PSV blowdown value can show if this may be the cause...

 

However if it would be the cause, you should change the PSV type to pilot operated one...It can tolerate that low difference between set pressre and operating pressure without mentioned problem...

[Lowflo]

William, It's not at all surprising to hear that this PSV leaks and doesn't close tightly after venting. The operating pressure is 93% of the set pressure. Furthermore, you say you don't have the capability to control this steam pressure. All types of safety valve require some amount of pressure reduction, below set pressure, in order for the valve to re-seat. A pilot operated valve (POSV) requires a minimal pressure reduction, while a conventional valve requires more (~ 5-10%). Installing a POSV will certainly help, but it must be a special all-metal POSV - one that doesn't have elastomeric o-rings like standard POSVs. The temperature is obviously too high for a standard POSV. Even with a POSV there are likely to be periodic releases when the system pressure operates uncontrolled at ~93% of set pressure

 

Engineers should understand that there are limits to our ability to solve problems easily and inexpensively. Clients sometimes present engineers with problems for which there are no good solutions, short of some fundamental/expensive changes to the system design. When they do, young engineers often feel like they're expected to perform magic to solve the problem inexpensively. The best response is cases like this is to inform the client that there's an option to improve the situation (install an all-metal POSV), but solving the problem may require fundamental changes, like implementing controls on the steam pressure or upgrading the equipment for a higher MAWP.

[chemsac2]

WilliamN,

 

I am presuming few things here:

1. Since PSV is on steam line and set pressure is 700 psig, system may have been designed for ASME section I.
2. Since ASME section I allows overpressure of 6%, blowdown would be around 3-4% (assumed same as API 526 PSVs which have around 7% blowdown for 10% overpressure) and considering +2-3%tolerance, maximum operating pressure should be about 7% lower than set pressure. This may explain why for operating pressure of 650 psig, set pressure is 700 psig.
3. If system is designed for ASME section I, 2x100% online PSVs (without isolation valves between system and PSV) would be present.

If above presumptions are valid, cause of leak can be damage to valve seat due to chattering. Chattering occurs in such a case as 2x100% PSVs would compete to relieve steam leading to neither of the PSVs opening completely and rapid opening/closing cycles.

 

If PSVs are damaged due to chattering on account of 2x100% online valves, those would have to be repaired in workshop or replaced.

 

Regards,

 

Sachin

[Bill B]

You stated this was a conventional valve, but I am not sure if this is an open bonnet (steam valve) type.  Several open bonnet valves are fairly tight at up to 95% of set point, or in your case 665 psig.  These are expensive compared to conventional gas valves (e.g. JOS, 1900), but might help.  The open bonnet valves are of course Section 1, but can be used in Section 8 service.

 A final option is the Dresser pilot, which is good up to 98% of set; I installed one of these on a 570 psig system; it required the external heat exchanger; don't know if it is good to 700 psig - contact Dresser.  These are Section 8 valves, but you said you were protecting piping so this should be Section 8.  I would not recommend this course of action unless you think the open bonnet will not work.

[CMA010]

If allowed by code you could use a supplemental loading system / air assist. In Europe maximum operating pressures up to 95% of set pressure are not uncommon in HP steam (generator) applications, however in these cases a supplemental loading system / air assist is installed.
You mentioned the problems started after the valve had popped, did you remove the valve after popping and reservice it?

[bevjones]

Change the PSV to a Crosby with a Flexi Disc or a Consolidated with a Thermodisc, they are quite good at staying tight at 98% in some cases.