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?

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. 

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.

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.

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