If you close a valve downstream of a sanitary centrifugal pump, in the short term, nothing particularly bad happens.  The impeller churns a bit and over time the product will heat up but nothing catastrophic happens.  That is not the case with a sanitary positive displacement pump. It will continue to pump, pressure will build, and at some point, the weakest part of the system will fail.  Either the pump will break (think snapped shafts) or something downstream will let go (Tri-Clamp joint, hose, valve, etc.)  So, if you are using a sanitary PD pump, it is prudent to have some sort of pressure relief built into your system.

We have been providing Waukesha sanitary pumping systems for many years and have used many different approaches to pressure relief.  Below is listed the most common techniques we have used.  They all work.  Which one you use is a function of how it best fits your individual applications.

Pump Front Cover Integral Relief
Waukesha offers an integral relief cover on its sanitary PD pumps. We don’t suggest our customers purchase the internally pressure relieved covers as they have quite a few disadvantages. We prefer other options.   Here are the pros and cons.

Disadvantages:

1. If it actuates, you do not have a visual method of checking it
2. They are not CIPable if they actuate, you need to clean them manually.
3. If they’re actuated, they internally vent from the discharge side to the inlet and can heat or damage the product due to the increased churning action.
4. They do not work well with high viscosity product, greater than 5,000 cps
5. Not full-flow relief. If there is complete flow shut off downstream, they are not guaranteed to protect the pump.
6. Elastomer limitations, Diaphragms are available in Buna-N (nitrile) or optionally Silicone.

Advantages:

1. Inexpensive.
2. Integrated for lower space claim/reduced plumbing.

Manual Pressure Relief Valve

Waukesha Manual Pressure Relief Valve

This is a simplest form of pressure relief.  You can stick a sanitary pressure relief valve on a tee downstream of the sanitary PD pump and set it to relieve at a specific pressure.  The discharge can either be sent to drain or re-piped back to the suction side of the pump.

Advantages

1.  Fairly simple and inexpensive
2. You can vary the pressure setting on the valve

Disadvantages

1. These valves have no indication so you have to set them by trial and error
2. There is a limit on how high a pressure setting you can have with these, 300psi is the highest rated valve we know of.
3. If the relief valve is piped to the atmosphere, you will lose product when it relieves.
4. If you pipe the valve to a the suction side of the pump, you have created a dead leg and another area that requires cleaning

CIP Bypass Loop with Air Operated Seat Valve

In a Pressure Relief/CIP Bypass Application the Inlet Stream Would Pass Through Ports 1 and 2, the Outlet Stream Through 3 and 4

This is an excellent choice if you plan on cleaning your sanitary PD pump in place(CIP).  This involves using a double cross body single seat sanitary valve piped to the pump.  The discharge side of the pump is piped through the lower chamber of the valve, the inlet side through the upper chamber.  The actuator can be air loaded to allow the valve to be pushed open at a set pressure, relieving upstream pressure, putting the pump into a recirculating mode.  During CIP, the valve can be opened to let some of the CIP fluid by pass the pump, allowing cleaning velocities in the piping to be maintained downstream.

Advantages

1. Solves CIP and pressure relief issues with one valve
2. No dead leg

Disadvantages

1. More complicated and expensive if you do not plan to CIP around the pump
2. Piping space may become an issue

Use a Clutch Type Coupling on the Pump

Autogard Clutch Type Coupling

There are several manufacturers of clutch type couplings designed to be mounted between the pump and the gear reducer.  In the case of over pressurization of the pump, the increased torque load causes the clutch to kick out, relieving the overpressure.  These can be include common loop powered 24V DC proximity switches with the coupling to send a signal that it has disengaged

Advantages

1. Fairly foolproof

Disadvantages

1. These are expensive
2. It is difficult to retrofit these to existing pump assemblies
3. If you do not include the sensor, you will not know when the over pressurization event occurs

Use an Electronic Pressure Transmitter Downstream of the Pump
This entails putting a sanitary pressure transmitter on a tee downstream of the pump.  Upon over pressurization the transmitter sends a signal to a controller that subsequently sends a signal that shuts down the pump.

Advantages

1. The hardware is fairly inexpensive given that you already have a PLC to hook the transmitter to.
2. You can a digital pressure gauge with a switch module to set a high pressure level and relay. This requires the use of a VFD, however, these are quite common with PD pumps.
3. Ideally, you don’t want a pressure relief valve to crack. By using an Anderson EN gauge as a high sensor, and a pressure relief valve as a “high, high” sensor, we ensure the valve only cracks under catastrophic circumstances.

Disadvantages

1. If there is any lag in the time of the pressure surge and the pump shutting off you may be in trouble.  Fluids do not compress.  Pressures can spike instantaneously.  In the split second it takes for the system to shut down major damage could occur.

So which is the best of these solutions for you?  It depends on your individual circumstances.  Like most things, there are tradeoffs to each solution.  We have customers doing virtually all of these techniques.  Contact us if you would like our opinion.  We would be happy to help.