Relief Valves for PD Pump Protection 2

[sshep]

Friends,

A plant we started up recently uses alot of PD process pumps. Per the company standard, relief valves were included in the pump discharge (to suction) to protect the piping against overpressure. The type of relief valve used is a hard seated Consolidated model 1911 which is typically of a type used for thermal relief. Due to various ways in which the pump can be blocked in during operation (including lack of operator awareness), we have continual chatter related problems: valves do not reseat, hydraulic hammers from chatter, etc. The Dresser alternative is to replace hard seat with soft seat (unfortunately it would have to be Kalrez), but even the Dresser rep on site admits that they do not really offer a good product for PD pump protection.

My proposal is to consider replacement with proportional acting relief devices from such alternative suppliers as Circle seal, Milton Roy, Swagelok, etc. The idea to replace the relief valves (even with a soft seat) is meeting with considerable resistance. There is even a vocal faction at this site that believes the only hope of mitigating this problem is to replace a dozen or so Milton Roy PD pumps with high head Sundyne pumps. I have seen these type problems before at other sites, and am fairly confident that we could make a far cheaper fix simply with a better style relief valve.

Do any of you know a good journal or on-line artical that discusses the selection of relief valves for PD pump service?

best wishes,
sshep

 

[unclesyd]

All our MR and BL pumps of any size are protected with rupture disks. We are pumping Nylon Salt so we have a surge pot installation on each system to take some of the load off the disk. The discharge of the disk is to atmosphere, drain. Most of the MR pumps have been changed to 5 high BL pumps.

The only relief valve we found that was suitable for our process was made by a company in Oklahoma for the oil patch. We needed one slightly larger than their highest capacity valve and they showed no interest in making it.

Addenda:
In your reference to using Sundyne Pumps you mention that you will require Kalrez O-rings for your process. Another very important consideration is the mechanical seal components. The reason I mention this is a project slipped through our review process and our company purchased 8 or 10 Sundyne Pumps and had installed several before any tests. It was quickly discovered that the WC stationary seal lasted only minutes. Neither the Ni or Co binder would work. Al2O3 was the only alternate at the time and Sundyne would not use it, reasons unknown.

 

[psafety]

How close are you operating to the relief setpoint?

 

[micalbrch]

Before you consider the installation of proportional pressure relief valves make sure that this is in compliance with any standard you must follow. There are standards (at least in Europe where I live) which only allow full stroke pressure relief valves.

Depending how close your operating pressure is to the relief valve set point an additional pressure switch with a set point below the relief valve set point will help, too. Its signal will turn the pump off before the relief valve opens. This will most probably not work in case an operator abruptly closes the discharge line but it prevents the relief valve from opening in "normal" situations.

I suppose your liquid is a slurry or contains at least some solids? If not, a spring loaded (adjustable) bypass valve which connects the discharge with the suction side is my last recommendation. Such a valve must be installed in addition to and not instead of the pressure relief valve.

I do not have experience with rupture discs but recommend not to select them by yourself but let a rupture disc manufacturer do the sizing. The disadvantage is that a rupture disc must be exchanged if it opened but that is (only) a price issue.

 

[Armen75]

First, have you determined why "valves do not reseat"

as per psafety, you might be operating too close to your set-point. With a traditionnal valve, you will see the valve lift as early as 90% of set-point. You might want to consider going to a pilot which can let you operate at higher pressures without openning.

Another consideration is your fluid... is it clean? is it causing the valve to stick open? Pilot with a filter, or Rupture disk, or rupture pin.

If the fluid is clean, maybe you might even consider installing a pulsation dampner (if you do not already have one) on the pump outlet to help "stabilise" the pressure... and by limiting the pressure pulses, you stay away from lifting the valve.

 

[sshep]

Thank you for all your replies. I would still like to find a good artical discussing PD pump relief design applicable in this situation.

To answer a few of your questions:

psafety, if the pump is not blocked in, the pressure deadband between the process and releif setting is 800kPa.

Armen75, The pump frequency is 120cycles/minute. When a blocked in condition occures (operator error at the pumps, frozen line, line-up problem downstream, etc), the relief valve could lift more than 7000x per hour. Reseat failure is inevitable at that frequency.

Micalbrch, what is a "full stroke" pressure relief valve? Do you have an industry standard to reference on any relief valve requirement and type. To my knowledge there is no code that addresses this, and the API standards (and my company standards) are weak on any detail. I will take your advice to make any new spillback bypass in parallel with the existing relief valve.

Unclesyd, thanks for your good advice as always and the heads up on the pump seal for a sundyne. A rupture disk is not a good option for us in this application.

Again, any reference to an artical that discusses this situation would be really appreciated.

best wishes,
sshep

 

[micalbrch]

sshep: A full stroke pressure relief valve opens completely at set set pressure and releases the full stroke volume of the pump. A proportional pressure relief valve opens completely at set pressure but starts to open partially (acc. to the flexibility of the spring) below set pressure. The European Pressure Vessel Directive does not allow proportional pressure relief valve. If your pump has a pulsation dampener, check the corresponding standard and find out what it requires about pressure relief valves.

 

[sshep]

Hey Micalbrch,

Thanks for the clarification. Although the small size of the pulsation dampaner could allow exclusion from the pressure vessel code (as piping like components), our own company standards do regard it as a pressure vessel based on a design pressure criteria of 5000kPa.

Taking your advice (and which was my plan anyway on further consideration), I am proposing to just install a second proportional valve in parallel with the existing code stamped PSV. This covers all cases- do you see any problem with that approach.

best wishes,
sshep

 

[moltenmetal]

If the relief valve is protecting piping rather than a vessel, I don't see a code reason why you couldn't use a non ASME proportional relief device such as a Swagelok or Circle Seal unit. We do this all the time. The smallest ASME relief valve you can buy is gigantic for PD metering pump flows- so large that its seat leakage even when NEW can affect the accuracy of your metering operation.

If these are Milton Roy hydraulic diaphragm pumps, you could leave your relief valve in place and re-set the internal hydraulic relief valve of the pump to a lower setting. Relying on this vendor-supplied relief valve alone for protection of downstream piping or vessels is not recommended. If these are packed plunger pumps of course that option is not available to you.

 

[sshep]

Hey Moltenmetal,

These are indeed Milton-Roy hydraulic diaphram pumps, but of process size flows. The capacities range from 0.7 to 7 cum/hr.

I think your idea to lower the internal setting below the convetional relief valve setting is a good one- simple and no cost. I know that the internal relief has an external adjustment which can be done on line by removing a plug to access the adjustment.

I am going to check it out with the rotating equipment engr now, and will let you know how that works out.

Thanks!
sshep

 

[micalbrch]

sshep: O.k. you have piston diaphragm pumps, not a pure piston or plunger pump. In this case follow moltonmetal's advice. That is the best and easiest way.

 

[Scotsinst]

You can also check to see if the downstream piping diameter is large enough. Since the diaphragm pump has instantaneous acceleration you can't use the nominal flowrate to determine required pipe size. There's a small chance the piping friction loss is sufficient to push the pressure above the PRV setpoint. It depends on the difference between the downstream pressure, the PD pump discharge piping frictional loss (instantaneous as the pump is discharging) and the PRV setpoint.
A pulsation dampener would solve the problem.