Our Micropac MW pumps are often piped into systems rather than supplied on a reservoir with suction tube already in place.
What suction tube is right, which is wrong, and how long can a suction tube be?
This is a basic question, but not commonly asked. Maybe hydraulic engineers get on with it from their experience. Our reservoir-mounted hand pumps already have their suction tubes in place. The question arises when you pipe a hand pump up to a reservoir using rigid tube or a hose.
What are the suction basics that allow a pump to “prime” and draw in fluid?
Let’s go back to basics on a pump, whether it is a hand piston pump, a diaphragm pump, air driven piston pump, gear pump or motor driven piston pump. There is a suction stroke where a vacuum is created by the pump to draw fluid in. Our YouTube video shows basic hand pump operation.
You will be very pushed to draw a high vacuum with a hydraulic pump. Working on absolute vacuum being minus 1 bar or minus 14.5 pounds per square inch, that describes the mass of a column of (say) water that could be supported. A figure of 10 metres is a theoretical maximum height but in reality, 6 or 7 metres might be a good starting point as discussed below.
The “cracking pressure” on the inlet check valve in the hand pump has to be deducted and on top of that the dead space in the design of the pump reduces the amount of vacuum generated. Again, basic physics. Those two will reduce the amount of the vacuum available. The cracking pressure is simply the hydraulic pressure needed to overcome the mass of the ball in a check valve and overcome the spring force, allowing fluid through. Be careful in fitting proprietary check valves in an inlet line. Many ranges might start at 5psi or 0.34 bar. That is a lot of suction vacuum to lose. Swagelok supply nice check valves with a useful 1/3psi or 0.022 bar cracking pressure.
The starting point for us would be a 50cc or 100cc Micropac MW series had pump. That will draw up fluid from some metres below the pump. That is a good solid amount of flow to work with. Drop to smaller displacements and the suction height is much more limited. Instinctively, a minute displacement piston pump is way less suction, hence why people use a boost pump or two stage pump to provide rapid prefill and also a positive head for the high pressure pump. On our MD or PSP two speed pumps and our two speed jacks, the low pressure stage provides filling up to a certain pressure, but also provides a permanent pressurised inlet for the small displacement high pressure stage. Watch our You Tube video describing this operation. On motor driven pumps, you find either two stage pumps with twin pump design or even a more basic “georotor” design.
Our Micropac PSP two speed pump uses a pressurised inlet for the high pressure stage. A nice design feature.
Can I use a large bore suction tube or a very long suction path?
There are two questions. You might need a long suction tube because of the installation. You might be taking an oil supply from another hydraulic system. What size of suction tube you need and at what point it is too large is a different question. A long suction tube with a large bore could work with three provisos.
It will take a lot of strokes to draw the fluid up. The big bore tube x a long run is a lot of fluid. That is basic hydraulics.
Secondly, you MUST allow a free flow of fluid out on the outlet at atmospheric pressure to bleed the unit. If you don’t, you are pumping compressed air into the outlet pipework and quite quickly, the pump will stop sucking in.
Thirdly, you are generating vacuum which is massively searching in a fluid system. If you have lots of fittings, be very aware of minute leaks that will degrade the suction effect. The same is true for a tube pushed over a hose tail. If it is a clear tube, sometimes you can actually see bubbles drawn in from the outside air by the suction vacuum. That is a real risk to suction lines.
What is the largest bore suction tube I can use for a hand pump?
We would always say that if it is a hand pump, there is no point in going for too large a bore inlet tube. Why? Even with one of our soft seat inlet check valves, there is the risk of the fluid level dropping slightly at the end of the stroke. More of an issue is pulling up the fluid over a length of larger bore inlet tube and leaving the pump for a few hours then finding there is air in the inlet and the system needs bleeding again. Using a smaller bore tube of say 9.5mm is always our choice for our Micropac hand pumps. On top of that, the chore of pumping a large number of strokes seems needless unless there is a really genuine reason to use the larger bore tube. In our world, having some very large bore tube in stock isn’t a good engineering reason if the system isn’t really right. For example, 14mm bore doesn’t seem quite right, even if it works fine.
Can I use a very small bore suction line?
Yes, you can but you will quickly find that the flow is restricted and the pump will not prime. Keep in mind that mineral oil can become viscous at cold temperatures. We always consult the Lee Products viscosity graph. You lift the handle and only get half a stroke. The rest of the movement is “dead” and the handle springs back. If you hold the handle at the end of the stroke against this strange “spring”, the flow will fill up the bore and provide a full stroke. This is a classic constricted inlet. It is either a small inlet tube bore, a blocked strainer, viscous oil/hydraulic fluid or a constriction in the inlet line. A constriction in the inlet line can be something as simple as using a small flow path banjo fitting. You need a decent flow path into any pump. On a Micropac MW series pump, we would say a 4mm bore inlet suction tube was too small. Even 6mm is small. 8mm feels better. Parker Hannifin will provide you with graphs for flow rates through pressure and suction lines, making it more scientific.
Will a foot valve hold the fluid in a suction line?
Yes, it will. The foot valve is an extra non return valve at the foot of the suction line normally in a reservoir which locks the column of fluid in the suction tube up to the pump. Even if you are only lifting a ball and don’t have a spring in this “foot valve” non return check valve, there will still be a cracking pressure to open it because of the mass of the ball. That in turn will reduce the effective vacuum generated by the pump. Instinctively, if you had a very large bore suction tube, you could find the fluid simply drained back to the reservoir. That’s why foot valves are used. A smaller bore tube would work better if the issue were fluid draining back to tank.
Why does my hand pump works on the bench but will not prime with a longer or larger bore dip tube?
The unit should prime although will take lots of strokes to pull the fluid up depending upon the pump displacement. You most definitely need a means of bleeding out the air when you commission the unit. As we have already said, you can’t just pump compressed air into the hydraulic system and expect the unit to work. Get a clean, bubble-free flow of fluid out of the pressure connection. Check out a blocked inlet strainer, a strainer with too small an aperture. Check there is fluid in the reservoir. And make sure any fittings are tight and work on the vacuum that the pump is producing. A very small displacement pump may need help priming. Don’t ask the impossible. And of course, don’t use too large a bore suction tube as the pump may not work its best.
Can I use a hydraulic filter on the inlet line? What’s the difference between a filter and a strainer?
This is an important point. Never use a hand pump without an inlet strainer. You just cannot have “muck” going into the hand pump and system. A strainer might be 100 microns or even larger. People often ask whether they can use a 3 micron hydraulic filter in the inlet line to a hand pump. The answer is no, as you will find the pressure drop through the filter element too high. People use a high pressure line filter on the output from a hand pump. Electric power packs invariably use a low pressure return line filter going back into the reservoir. That is a neat way of cleaning the fluid on a continuing basis.
What can Sarum Hydraulics offer me?
After over four decades of manufacturing our Micropac hydraulic hand pumps, we are always pleased to discuss applications for our pumps. Contact us.
Leave A Comment