Can a pressure surge "pass thorugh" a plate and frame HX? 1

[MortenA]

Anybody who will dare en opinion with regards to the question: Will a pressure sure (generated by closing a downstream valve) "pass through" a P&F HX? I want to install a bursting disc upstream the HX (for reasons NOT related to the surge) but i dont want the disc to burst if an occational pressure surge should go just above the design pressure (could have happended and the piping was OK with it but the disc would not be….)

Best regards, Morten

 

[RVAmeche]

You say you don't want the disc to burst if surge gets above design pressure...so the set pressure won't be the design pressure. What will it be?

The IOM for Alfa Laval plate and frame heat exchangers warns that pressure surge/water hammer will cause damage to the equipment, which is what I expected.

 

[EdStainless]

Most P&F units don't like even modest over pressure. It causes enough plate deformation that sealing can become an issue and it can also change flows and heat transfer values.

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P.E. Metallurgy

 

[MortenA]

Ed, thanks. Since we already have operated this system for many years i guess we would have observed if surges of any significanse we common.

Vhers, Morten

 

[1503-44]

To answer the original question. Pressure surge will go through pretty much anything that it can't sufficiently expand, or explode, to accomodate the accompanying surge volume.

 

[EdStainless]

Morten,
I was in a plant and they were going to replace a P&F HX. In preparation they very carefully measured the performance of the existing unit. The had flow, pressure inlet and drop, and Tin Tout for both streams. A young engineer on the project was looking at the numbers and began to ask why the operating point was so different from the original acceptance testing. The plant engineer insisted that nothing had changed. One side had a PD pump so the flow was exactly the same, but the pressure drop was different. The other side had a centrifugal pump, everyone had assumed that the shift there was due to pump wear, but as they tried to adjust the flow it became obvious that at the original pressure drop they had a different flow. So of course the delta Ts were different from original as well. All of these shifts were in the range of 5-10%. In the end they cut the unit open and found distorted plates that had been restricting flow on one side and allowing flow to short-circuit on the other. This was from repeated slight over pressure events.

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P.E. Metallurgy

 

[TiCl4]

but i dont want the disc to burst if an occational pressure surge should go just above the design pressure

It is a poor idea (at best), possibly a violation of code requirements/jurisdictional law (is the Hx stamped?), and a safety issue if you provide a disc that has a stamped burst pressure above design pressure of the heat exchanger!!! The design pressure is there for a reason! The shock-wave will transmit directly through the heat exchanger; if the shockwave comes in the hot side, the plate will flex and compress to the cold side, exposing both sides of the heat exchanger to the shock.

The website link below has an explanation of instantaneous valve closure and slower valve closure. Different equations govern the pressure spike depending on the valve closure time versus the wave propagation speed. I have seen flanges burst from a quickly-closed valve on a 700 ft long pipeline! Please note the sensitivity of the pressure spike to valve closure time. If the problem is an automatic valve, you can install a bleed restrictor that slows down the closing time and prevents this issue in the first place. If it is a quarter-turn manual valve that is closing, it is likely that closure should be treated as instantaneous.

If the pressure spike is above the design pressure of your system, but you don't want the rupture disc to blow, look at installing an appropriately sized surge suppressor near the valve that can initiate the shock-wave. Surge suppressor manufacturers will have sizing equations to tell you the size of suppressor needed to appropriately dampen the shockwave.

https://www.lmnoeng.com/WaterHammer/impulse.php

 

[georgeverghese]

Bursting discs may rupture at pressures as low as 80% of calculated bursting pressure( with temp derate included). So it sounds like you have a problem even without the surge event?

 

[MortenA]

We decided toinstall a PSV instead.

 

[LittleInch]

Where? U/S or D/S the HX? Sounds like the HX really doesn't like pressure surges.

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

 

[MortenA]

as i have tried to explain many time - the device(now a PSV) is NOT for protecting the hx or piping -because we know that they have not seen damage. PED allows a 10% overpressure from upsets (surge would be something like that) We dont _know_ if we even have a surge and the fact that the HX look fine seems to indicate that we dont.

Its for a special scenario that plays out like this: Due to an error in the BPCS a valve upstream and downstream the pump closes- but the pump continues to run. The pump heats the water causing the pressure to rise until the casing bursts potentially killing somebody near it. In order to have a mechanism for reducing the risk for this scenario we can employ a number of different solutions - in this case either a BD or a PSV will be the easiest. At the end of the day we choose a PSV (partially because we had a suitable one lying around that we did not have another use for...

If you consider this impossible then take a look at this:

https://www.aiche.org/ccps/resources/process-safety-beacon/archives/2013/august/english

 

[georgeverghese]

What type of shaft seals do you have on this pump? The seals would have given way long before the casing ruptures in this blocked in - overheating scenario.
The overpressure due to surge scenario at the HX seems realistic enough. But a controlled closing speed on the discharge side SDV would also be an adequate alternate mechanism to the PSV to address this case - also use a wafer check valve on the pump discharge to give you better flow turndown compared to a swing check.

 

[MortenA]

im not going to argue wrt the scenario (check the link once again)

 

[georgeverghese]

As suggested in the link, a neater solution to this blocked in operation ( which appears to be downright reckless if you ask me), would be install a TSHH on the pump discharge, and / or interlock one of the isolation valves to the start sequence of the pump. Avoid adding PSVs' all over the place to address poor operating practice if you can.
You can also check with the shaft seal vendor if these seals will hold up to pressures approaching casing rupture pressure. Casing rupture pressure would be much higher than pump mechanical design pressure.

 

[MortenA]

thanks and i agree that it would be a better solution (instrumented) in this case, but that would require some extra DCS stuff that would crank up the price. Wrt to the scenario i dont disagree that the probability is low but its a case we have to consider (no exceptions) by demand from our global Engineering department.

 

[georgeverghese]

The failure of the pump in the link, I suspect, is more likely due to severe mechanical stresses on the pump casing and nozzles resulting from high vibration attendant with running at blocked in dead head conditions. This happens on pumps with driver rated power in excess of approx 100hp. So a more appropriate trip loop would be an FSHH and not TSHH.

 

[waross]

Hi MortenA. Check your mail. Particularly old, seldom used, mailboxes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LittleInch]

In particular the one listed here in "my profile"

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