PSV lifted below Set Pressure 4

[Rubje]

We have a PSV on a boiler that is set to lift at 465#. It lifted at 465# (process upset). It lifted again about a week later at only 430#. We assumed it had an issue after the first lift so we removed it and shop tested it, expecting to find an issue. Upon removal we didn't see anything that was stuck in the seat of the valve.

However the shop test was perfect, lifted normally near 465#.

We already checked the lift at 430# and it was indeed actually 430#. We have multiple pressure transmitters to confirm the pressures.

I'm stumped, I've never seen this before. What would cause this? What am I missing?

 

[LittleInch]

Set pressure?
Type of valve?

Was the any transient pressure?

But sounds like it's within the normal range of a spring relief valve.[pre][/pre]

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

Describe the event and it's hydraulics in relation to the PTs and the PSV. A well labeled/dimensioned picture/drawing/sketch showing all would be great. Was the PSV conventional, bellows, pilot operated? How long was the PSV open? Could back-pressure on the discharge have changed?

Good Luck,
Latexman

 

[LittleInch]

It seems more worrying to me that you have had two process upsets in a week which resulted in a PSV lifting. These are safety incidents which should be investigated and treated as if they were failures.

I would concentrate on that first before worrying about the valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pierreick]

Hi,
As stated by other, I believe your PSV is operating in its normal range, let say -7%+10%.
I don't understand your logic, you dismantle and check the PSV and you did not mention anything about the Pressure transmitter and connection to DCS (I suppose). When was the PT calibrated?
My 2 cents
Pierre

 

[MJCronin]

Agree with LittleInch .... He wants to know WHY you are operating so close to the setpoint ?..... Me too !

MJCronin
Sr. Process Engineer

 

[Sabeau]

Would help to have a more information on the actual application and service too. I know you said it's on a boiler but in cases where a PSV is normally seeing liquid and then is exposed to a gas this can cause early valve lifting in the range you're talking about.

 

[Rubje]

Thank you everyone for the responses:

@littleInch: Set Pressure is 465#, conventional spring loaded valve. As for transient pressure I'm not sure about that. Any idea how to confirm?

Technically, there was one process upset since the second relief was at a lower pressure below the set pressure. The issue that we are dealing with is that a valve failed on a line that provides steam to a main user of the steam system. This user approximately uses 30% of the steam in the system. The pressure rise was so rapid that the pressure alarms didn't end up being helpful for the head operator.

We are in the middle of replacing the failed valve and perhaps we should take another look at our PHA.

@latexman: PT is in the vapor space of the drum. PSV is also off the top of the vapor space but about 15 away from the PT. It's a conventional valve. As for duration maybe a 1-2 seconds. It's open to atmosphere so there shouldn't be any issue with backpressure. I will ask the mechanic if he saw any obstruction in the tail pipe.

@pierreick: our operators taking readings every shift on various pressure gauges. The pressure gauge on the drum is always compared to the pressure transmitter and they both match perfectly. There is also a redundant PT and this also matches the other PT and pressure gauge.

@MJCronin: As mentioned above, a vale to our main steam user has been sticking and acting up. this is why we been seeing higher operating pressures lately. We are in the process of addressing this failed valve.

@Sabeau: What information exactly are you looking for?

 

[The Obturator]

Can you advise the valve manufacturer, valve model no and size? Also what is your normal operating pressure?


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[LittleInch]

Conventional spring operated relief valves typically claim about -8% from set point for full tightness, so in your case about 430 psi.

So it's at the bottom end of what would be expected, but if your transient pressure / time of steam release was only a few seconds then there may have been some delay time on the 465 psi release or maybe it just loosened everything up a bit and didn't reseat exactly the same.

It all depends on how fast your control system is and how often it records the readings, but the historian on your control system would be interesting to see what sort of time your pressure rise was.

I know we're being a bit critical here, but loosing 35% of your demand should be well within the capability of the system to control pressure before you hit your ultimate safety device. It tells me your system is operating too close to the limit and /or has a large thermal inertia which somehow needs to be controlled a bit more or incorporate some other steam shedding before you hit the main pressure relief system setting. In general an alarm which doesn't allow the operator to actually do anything before it hits the next stage is just a noise making annoyance. Not sure exactly, but it should give the operator at least 60 seconds or more to do something and that change to have an impact for an alarm to be of any use.

I'm glad you're taking it seriously - if the design is quite old, lots of things assumed then may have changed and it needs a new look at the system to reduce risk and keep everything going within the design limits of the system. Setting of alrm points is often an art rahter than strict science once you have operating experience as you don't want false alrms, but you need time to do something.

But the valve - looks just about within spec to me. If you need something with lower margin then you need pilot operated. Generally 3-4% negative tolerance. Don't be tempted to increase the set point though beyond the MAWP. That's a Bad move.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

1-2 seconds? That may not have even been a full lift release.

My company's guidelines:
"Point at which device will begin to leak when in good condition, % of opening pressure"
90% for PSV
"Recommended maximum equipment operating pressure when using PSV, % of equipment MAWP"
90% (metal-to-metal seats)

430#/465# = 0.925 Too high?

Good Luck,
Latexman

 

[Rubje]

@The Obturator: I need to check on that info. Normal Operating Pressure is around 400#

@LittleInch: Thanks for that detailed response! I'm going to sit down with operations and go over some of the comments you provided. I know this isn't directly related but the boiler is a single element control and I been pushing for a 3 element control for a while now. Maybe this incident will give me more support to get approval for a 3 element control.

Again, I appreciate all the support everyone!

 

[LittleInch]

Well based on your relief valve pressure and type of valve a NOP of 400 psi sounds too close to the limit if your flow can change rapidly.

You might need a different type of valve and a better control system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Sabeau]

@Rubje You answered my question, the PSV is the in the vapor space. I was considering if the setting media might have something to do with a low relief.

To Latexman's point, even 90% operating to set pressure is probably too close for a SOPRV (normal operating pressure)

 

[georgeverghese]

To enable a longer reaction time for the control valve / some operator response time after the PAH alarm, you could add a volume buffer tank on the 400psig line. This may be in the form of an oversize length of pipe.
Sometimes, when a quick response is required from the control valve, the instrument air supply to the CV may be too small and acting as a choke - check if this is so. Or the field lubricator for this CV may be flooded with water.
If instrument air supply is not an issue, adding a volume booster to the control valve can enable quicker response.
Note a conventional PSV will start to simmer at 90% of set pressure. And reseat pressure is typically at 93% of set pressure.
Else switch out to a pilot operated PSV which should allow operation at up to 95% of set pressure at least - expensive option.

 

[Sawsan311]

First ensure your PSV inlet pipe is upstream any demisting device otherwise it would have been responding to an artificially lower set pressure.
Spring loaded type should not have been selected due to:
1. blowdown is maximum 7%-10% as per ASME SEC VII code and hence operating pressure shall not exceed 90%-93% of the set pressure. Care shall also be taken to the set pressure tolerances discussed in non-mandatory appendix-M of ASME and UG-126.
2. Without changing the PSV type which is obviously a design selection issue since your operating pressure should be less than the PSV set pressure, consult the manufacturer to adjust the blowdown seats to allow smaller blowdown through the blowdown seats (if and only if he confirms no violation from the code)
3. Check the backpressure downstream the PSV, usually it is difficult to tightly seat the PSVs under low backpressure, this is where the differential cold test pressure comes into picture i.e. CDTP was adjusted consider higher superimposed backpressure than the actual in reality.

 

[shvet]

I do not understand what all of you are discussing ))) Set tolerance, really? )))

Extracted from para. 2.11.1 CCPS's Guidelines for Pressure Relief and Effluent Handling Systems 1998

 

[FacEngrPE]

Historians often do not detect the peak of due to the data being recorded at the designated intervals. Unless the intervals are really short.

It would not surprise me to find that your boiler tuning is too slow to catch a pressure upset from a downward 30% step load change. Making the boiler master pressure control fast needs to be balanced against the need to minimize firing rate hunting.

If fast steam demand change was a regular problem, I would add a dump valve with a muffler, and control it from the boiler master as an additional boiler. A dump valve does not have the physics restrictions on rate of change that a boiler has. This is similar to using a pilot operated relief valve, but is better as it avoids frequent lifting of the PSV.

 

[Rubje]

@LittleInch, Sabeau: Thanks for pointing this out. I'm going to have this fixed.

@ georgeverghese: I haven't heard of a volume booster believe it or not. This may fix some other issues we are dealing with in the plant. thanks!

@Sawsan311: Good advice thank you

@shvet:

Wow those numbers are actually pretty surprising.

@FacEngrPE: This is interesting thank you