Solenoid operated Vented Relief valve 1

[pob786]

Hi, I am working on a system which comprises of a 65 litres/min Constant pressure axial piston pump (set at 175 Bar), together with a 110v AC Solenoid operated Vented Relief valve in the system set at 200 Bar, being utilised to discharge "walking trailers" via associated pipework, hoses and Quick Release couplings. The problem being that occasionally the system is started with the Quick Release couplings dis-connected so that the pump is "dead headed" at 175 Bar when the Vented Relief valve is energised. The pulsation in the pipework upon energising or de-energising leaves a lot to be desired?
Does anyone know of a manufacturer of a Time/Speed controlled solenoid vented relief valve to prevent/reduce the pulsation?, or even suggest an alternate method of control?
Thanks.

 

[Oldhydroman]

I kind of agree about turning off the flow not the HPU but, for reasons of "safety" (and to pamper to the ill-informed and the risk-averse members of the board), some organisations do require the HPU to be turned off completely before connecting and disconnecting the QRC's. It's not nice for the pump or for the electric motor to be turned on and off too often. For the electric motor in particular there are real issues regarding the maximum permissible number of "starts per hour" (might be as low as 6 but it depends on if its a DOL, star-delta or soft start). At the end of the day though, the pump and the electric motor do not have a God given right to be treated with respect. If the end user has some sort of operational policy which shortens the service life of the equipment then that's just fine ... because it is his equipment to do with as he pleases.

I suppose the supplier of the HPU might want to say to his customer something along the lines of "If you are going to treat the equipment that badly then don't expect any warranty cover" but it's a tough economic climate and the supplier might just have to say nothing and hope for the best.

There's not many bells and whistles on this HPU, for example, I can't see a cooler, a dual level float switch, any over temperature thermostats, a filling QRC, a tank drain valve, a limit switch on the suction isolator etc. Would it be fair to say that the HPU has been built to a price because of budget constraints? That might be why there is no DCV.

Don't get me wrong, "Horses for courses" is what I say, but, speaking personally, I often have to shrug my shoulders and hand over a system design which is more "Flybe economy class" than "Singapore Airlines business class". Is it possible, however, that this system is just too basic and that it will cost the end user much more in the long run?

In the link below (if it works) is a scheme I came across once to give a simple co-ordination between the unloading relief valve and the pilot operated DCV. I'm not suggesting it for the solution here because it is more suitable for a fixed displacement pump (but, pob786, if you like it then feel free to adopt it). I've drawn the DCV symbol out with all it's detail because that allows you to see the unusual choice of pilot valve. Also note that the pilot supply to the DCV (port X) comes from the vent port of the relief valve.

When you energise either one of the solenoids just the bypass pressure is applied to the DCV main stage spool. That's not enough to get it to shift but the vent line is no longer connected to tank and the system pressure will now begin to rise. The spool shifts a bit (but not too fast because the pilot line flow is limited to a 3 bar drop across a 0.8 mm jet). As the spool shifts the pressure begins to rise, and as the pressure rises the spool shifts more, and so on ... until the DCV spool is fully shifted; and only then can the supply pressure rise to maximum.

When both the solenoids are off the T port of the DCV connects to tank with absolutely no flow going through that line. If you had QRC's on the connections to the actuator then, when the DCV is in neutral, there will be no pressure on either of the couplings, not even any back pressure from a flow to tank. This would allow the QRC's to be connected or disconnected easily and with minimal leakage. You might even be able to convince a Luddite that it would be quite safe to operate the system like this.

DOL

 

[hydtools]

Need to ask OP if forward and reverse flow are required or not. If not, then only two of the three positions of the directional control valve would be used.

Ted

 

[Oldhydroman]

Absolutely, but I put the circuit up the way I did as an example of bi-directional flow for anyone else that might be interested. I took the view that if the reader could fathom the meaning of the diagram I attached then they would also see the ways in which it could be re-configured for the exact requirements of whichever machine was being powered.

In this application I suppose it all comes down to a choice of how you want to unload the pressure compensated pump. You could have full flow at low pressure (vent the relief valve) or full pressure at [nominally] zero flow (de-energise a DCV). When you are operating the walking floor mechanism then the pump will probably need to be on part displacement. If your unloading scheme previously had the pump on full flow then it has to reduce its displacement to get to the right position and this is where the original pressure peak came from. If your unloading scheme previously had the pump on zero flow then the displacement has to increase to get to the right position and you will get a dip in pressure (and maybe a small overshoot just as the pump homes in on the exact position).

Of course, whichever option you choose then you get the opposite effect when you unload the pump. There will be a pressure peak as you try to force the pump back to zero displacement and a pressure drop as you let it go to full displacement (which isn't a problem because you let the pressure drop anyway by venting the relief valve). The pressure peak isn't so much of a problem either if you have a DCV because the pulse will be contained within the HPU and not experienced by the larger external system. A soft switching valve would be nice in this application.

Ted, for the benefit of others who may be following this thread, do you think there would be an interest in exploring some of the ways that a two stage DCV can have its switching time increased? Or should this thread now be allowed to taper off?

DOL

 

[hydtools]

I'm letting this thread go. Suggestions have been offered.

Ted

 

[hydtools]

I guess I cannot just let it go. Pressure pulses result from rapid changes in velocity, flow. When the directional control valve is actuated and no couplers are connected there will be a slight variation in pressure since the piping to the couplers is full of fluid, velocity is zero and pump flow is zero. There will be a small surge as the previously unpressurized part of the system is pressurized and component expansion occurs. No harm, no banging. De-energize the directional control valve and make the coupling connections. There will be a sudden pressure drop as the pressurized part of the system to the coupler connects to the return to tank line.

If the couplers are connected when the directional control valve is actuated, fluid piping is full and flow is zero there will be a small surge as the previously unpressurized part of the system and acutators is pressurized. Starting the load from rest is a zero-velocity event, flow will increase as the load is accelerated up to the point where the flow control takes over control of flow.

Now I'm finished. Very informative replies, DOL

Ted

 

[hydtools]

The bane of engineers, cannot stop thinking about a problem.

pob786, try what may be an easy test. Change the sequence of the relief valve and pump. Energize the relief valve before starting the pump. If the couplers are not connected, the pump will run several revolutions as it starts and reach the 175bar pressure compensation setting, then continue to run up to speed at near-zero torque load as the swashplate will be nearly flat. This will be a low load startup rather than coming to full flow before the relief is energized after the pump start. The pressure rise will be that of the pump reaching 175bar pressure which will be slower that slamming the relief valve shut and suddenly stopping the bypass flow. There should be no hammering and banging. The shutdown sequence would be: turn off the pump then de-energize the relief valve to drop system pressure. No new components, just the system as is.

Ted

 

[hydromech]

Ted,

I stopped contributing long ago...

There is only so much help that can be offered on this forum. People can only take the advice and use it or ignore it.

There can be a tendency for people to "show what they know", which can obscure the real solution to the problem.

This forum can only offer advice, if people want a solution, they might have to pay for it!!!

Adrian

 

[Oldhydroman]

But the advice should be the best advice that's going. For what it's worth, my advice would be that you tread very carefully when attempting to start an electric motor with a pressure compensated piston pump where the relief valve has not been vented.

The pump will be on full displacement right from the beginning. It won't start to de-stroke until the pressure rises to the setting of the compensator. At that instant both the pressure and the displacement will be at a maximum which means the required drive torque will also be at a maximum.

If the motor is being started with a star-delta starter then there could be insufficient torque output from the motor for it to be able to complete it's run up and it could stall, or else creep along at low speed. The current will remain at 3-4 times full load current for the remainder of the star-running period. If just creeping round then the motor might even come to a stop during the time delay between the star and delta contactor energisation. Finally, the delta contactor will come in and you will perform a DOL start on a fully loaded motor (expect 8-9 times full load current).

When you turn off the motor without venting the relief valve then the rotating parts of the pump and motor will come to a sudden stop and the excessive stress in the flexible shaft coupling might shorten its life considerably.

Some people worry about the effect on the pump when starting and stopping on full pressure ... but I take a philosophical view on this. If it was a hydraulic motor with the same rotary group then the application might require it to start and stop on full pressure and you wouldn't be concerned about it.

Sorry gents, I was going to consider this thread well and truly talked out but I felt I needed to throw in that particular caution.

Over and out.

DOL