an exploded drawing of a Micropac check valve showing all the part including a spring, seat, poppet and seals

A pump check valve has complexity and needs to be right.

How to design inlet and outlet check valves for your pump. 

Are the check valves for a pump the same as catalogue hydraulic check valves?

We would say no. Lots of features are the same but there are some big differences.

Are the inlet and outlet check valves the most difficult thing to design for a pump?

Yes, they probably are. They have to work when you want them to, hold pressure and be long lasting. That can mean millions of cycles.

Is a check valve the same as a non return valve?

Yes. The US call them check valves, which is an easy name for writing. The UK and lots of other languages call them “non return valves.”

Is an air check valve the same as a fluid type?

Yes, up to a point. Air has a very low viscosity so you can get a lot of flow through a small orifice. The downside is that air or other gases are very demanding on sealing and will need an elastomeric seal. A hydraulic fluid check valve may use an elastomeric seal for very good pressure holding but can use a polymer plastic valve seat or even a hard metal seat. It all depends on the fluid and the requirement on pressure holding. Hydraulic check valves are our thing.

Is a pump check valve just a ball on a machined drill point? What is the big deal?

Yes, a check valve can be as simple as a machined seat and a ball “peened” lightly to form a sealing ring. There is detail to design. Spring load? Dimensions? Finishes.? Limitations on ball check valves? A good designer will ensure that when a fitting is removed, the check valve does not fall out. That is very annoying in any product. There is a lot to sort out for a good product.

What are the low pressure and high pressure limits for check valves?

If you are buying them in, the maximum pressure will be stated. The lowest pressure at which they will operate is a different matter and relates to the cracking pressure. You need enough pressure to lift the ball or poppet against a spring load. The check valve is effectively working like a relief valve at low pressures, so do pay attention to cracking pressures if you are working at purely low pressures. High pressure limits are more difficult on valves that you are designing yourself. There are a lot of considerations here. For a ball check valve on a polymer seat, you are limited on load. A ball check valve relies on a narrow sealing area for optimum operation. Excessive load will spread this sealing area out, ultimately over-indenting the seat. That isn’t a good place to be if a valve is going to operate for a large number of cycles. Our experience also tells us that larger contact areas a more susceptible to contamination, although we can’t describe this formally. A poppet with an elastomeric seat has pluses for high pressures. The area is formed by a cone, so the load is spread evenly. If you are using a seal, beware extrusion at very high pressures.  Lots of high pressure check valves use a hard ball like tungsten carbide seated on a hard seat. The size of the ball and orifice is trimmed back as small as possible, simply because the loads become enormous as the diameter increases. We think high pressure check valves are specialist. Look what other people do.

What’s the difference between inlet and outlet check valves?

The inlet check valve is on the inlet suction path into a pump and the outlet check valve is on the outlet path of the pump on the delivery side. Both can be similar in design, but each has its own essential features.

Look at the function of each if our pump is something simple like the barrel of a Micropac hand pump. On the suction stroke of our simplest single acting hand pump, lift the handle to increase the volume in the barrel and draw fluid in. On the pressure stroke, the operator is pushing the piston rod back into the barrel and reducing the volume. Fluid is pushed out. Look at our YouTube video (embedded below) which explains this action and adds in the difference between single acting and double acting pumps as well. The change in volume is the same for any reciprocating (=moving backwards and forwards) pump. You will need two check valves, one inlet (or suction) to let fluid in on the suction stroke and one outlet check valve to let fluid out on the pressure stroke but not back through on the suction stroke. You’ve got the basis of a pump which will draw fluid up a hose from a reservoir and push it out down a pressure or delivery hose.

So, what is special about an outlet check valve?

There are a few musts. The valve must seal ready for the suction stroke. You cannot have fluid flowing back into the pump from the delivery hose when you stop pumping. If it is an oil pump, a ball on a metal seat may well be perfect. Run on water or low viscosity fluids and you need elastomeric sealing or at least a soft seat.

For 40 years, we have used a dedicated poppet check valve with a replaceable seat. The poppet has an o ring seal. Invariably the valve will have a spring in it to return the ball or poppet into the valve seat. There are check valves that simply lift a ball or poppet and rely on gravity to pop it back into the seat. You do need something quite positive to get the valve resealed at which point the pressure in the system should hold it shut. There is nothing worse than a lazy outlet check valve with a lag before it shuts. An associated problem is a valve where the ball or poppet travels too far. This can make the valve lag. Good designers always control the amount a check valve opens both to make the action quick and also to stop a spring being over compressed and ultimately fatiguing. Do keep in mind that the orifice of the valve has a certain area and the ball or poppet only needs to open up the same area as it lifts. It is surprising how little this can be. Do the arithmetic. Our final thought on outlet check valves is cracking pressures. This is the pressure at which a check valve opens. The outlet check valve cracking pressure can be higher than the critical low cracking pressure of the inlet check valve but be careful. If you are priming your pump, the first few strokes will be air pumped through to pull fluid up a suction tube. If the pump simply can’t generate enough air pressure to lift the ball or poppet, your pump isn’t going to self prime. Having to instruct an operator to fill a barrel with fluid isn’t good. We might be looking at cracking pressures on our own valves of 0.3 bar or less.

A non return valve assembly from a Micropac pump. A seat, guided poppet, a spacer tube, spring and threaded retainer.

Our Micropac outlet check valve uses a replaceable seat and a guided poppet.

an exploded drawing of a Micropac check valve showing all the part including a spring, seat, poppet and seals.

An exploded drawings of our Micropac outlet check valve. They are all our manufactured parts, as this isn’t an off the shelf component for our needs.

Inlet check valves can be very difficult indeed to design.

The same basics apply to the inlet valve in that the valve has to be leakproof or very low leakage. The orifice size on your inlet check valve must be large enough to allow the inlet shot of fluid to be unconstricted even with the most viscous fluid and also at maximum pump speed. Get this wrong and the pump will cavitate, giving a “dead” portion of the stroke.

Instinctively, you might expect an inlet check valve to be larger than the outlet check valve. We think it is probably caution on having it big enough so you don’t have cavitation at high flow rates or with more viscous fluids. There is an argument that a larger area of valve will lift more easily under vacuum, but we think the former point on cavitation might be the big issue. As with the outlet check, always control the maximum movement of the ball or poppet. Spring load is the most difficult challenge. If the load is too low, the valve will be sluggish to snap shut. Get the load too high and you may fail to draw any fluid in on that crucial first stroke. The pump is forced to draw a vacuum, which has to be high enough to lift that inlet ball. Our cracking pressures are as low as 0.05 bar. Some pumps like very small displacement gear or piston pumps address this by running a low pressure pump of some sort which can provide a source of power for “fast advance” on actuators but also provides a pressurised inlet for the pump. Even a small positive pressure on an inlet check valve will prime the pump instantly. Some 700 bar piston pumps have truly minute displacements per revolutions and really benefit from a pressurised inlet. Even our own PSP two speed hand pump employs a similar pressurised high pressure stage inlet as used on the Tangye Hydrapak, or the Hi Force HMP and MHX units. It is a clever configuration that gets around struggling to persuade a small displacement high pressure pump to prime. We did a useful application note on these ideas. One final thought is to be aware of how much viscosity changes with temperature. We have always been fans of Lee Products’ viscosity graph for a massive range of fluids. Some everyday media get remarkably viscous and indeed reach their pour point at what isn’t actually very cold in Canada or Siberia.

Where should the check valves be situated relative to the pump barrel?

We would say as close as possible.

Indeed, your whole design intent might be to get the valves incredibly close, particularly the inlet check valve. If there is a passage between the pump chamber and the check valves, making it large enough diameter to cope with the flow at the highest viscosity, but no larger. When priming the pump, this volume is effectively added to the barrel volume and governs what vacuum you can generate to lift that inlet check valve and pull fluid in. Be very careful.

Is there an alternative to ball or poppet check valves?

There are alternatives.

Lots of low pressure pumps use simple flap valves. Sometimes these can even be incorporated into a pump gasket for a very simple design. Wire springs are often used on simple flat flaps as trusting a rubber flap to be pushed back onto a seat purely by flow isn’t always robust enough. The rubber can take a permanent set and not be shutting off the seat. A more sophisticated moulding like a “duckbill” valve or the triple Jabsco valve does address this issue. Those are purely low pressure.

A black moulded rubber Jabsco non return valve with a flange and triple orifice

A Jabsco “Joker” moulded check valve Copyright Jabsco

TMC Toilet Flap Valve

Flap non return valve from a TMC marine toilet.

We would look at how people do it. Look at marine bilge pumps and old automotive fuel pumps. Another option is mechanically opening up or closing an orifice without using balls or poppets that move. Look at a swashplate pump and you will find a valve disc that rotates and opens or closes the correct passages. That is quite exotic engineering unless you are making a lot. Another totally different approach replicates the human digestive system. A pump like a peristaltic pump basically has no check valves. Rollers simply squeeze “charges” of fluid along a flexible hose. That is limited to quite low pressure but is mighty clever. Peristaltic pumps thrive in the right application.

Peristaltic pump diagram showing loop of tube and the two rollers moving round, forcing a slug of fluid through the pump

Operation of a peristaltic pump. Image Copyright Watson Marlow

How do check valves fail?

Look at a nicely engineered check valve like the Swagelok products and you will see that the poppet is guided very accurately. If it isn’t, you can find that a poppet or ball can be cycling in an ellipse and start “tracking” which can wear an oval. Ball check valves can do this. Springs can fatigue and break up if they are over-stressed. This is a disastrous failure mode, as fragments of spring can contaminate a system. Contamination is always a risk for check valves. Consider “last chance” strainers for larger and very destructive contamination. That is the extreme but do keep in mind that low leakage does require clean valves.

If Inlet and Outlet check valves for pumps are this awkward, why not just buy them in?

Commercial hydraulic check valves invariably have cracking pressures that are too high. Look to instrumentation valves and you can order Swagelok check valves right down to 1/3psi. BIS is another good UK supplier who we use.

Check Valve section showing compression fittings either end a the internal parts, including a poppet and spring.

A Swagelok check valve with nicely guided poppet

Image Copyright Swagelok

What check valves do we use in our Micropac pumps?

They are all our own. The medium is largely what drives the valve we use.

Our Micropac pumps for only mineral oil generally use ball check valves. We use replaceable valve seats on a lot of our kit so that you can service units. The low cost alloy/carbon steel MT cartridge pump and most of our jacks use ball checks. Move to multi-fluid pumps like the MW-3 single speed, the MB drum dispenser/low pressure pump or the MD-Two speed pump and you will find soft seat and poppet check valves for low leakage and long life.

3/4 angle view of our two speed hydraulic handpump

Our Micropac MD series 1000 bar 316 stainless hand pump

After over 40 years, we know a lot about hand pumps and the check valves that go into them. Look at our Micropac range or contact us to discuss your requirement. We are the experts.