Troubleshooting Insufficient Flow in Closed Center Load Sensing Systems

    In this article I would like to discuss two malfunctions, which involved classic closed center load sensing systems. Both malfunctions had similar symptoms, but completely different causes - a matter worthy of discussion on the IH pages.

    Malfunction one - a troubleshooting episode, that I already described in the battlefield section (original post here), but would like to recall again in this post. Very briefly - a forest crane, equipped with closed center load sensing system, was getting slower as the oil temperature rose. The malfunction was diagnosed as the main pump wear, however after the consequent overhaul the symptoms remained unchanged. A more detailed analysis revealed malfunctioning pressure reducing valve that was supplying pilot pressure to the proportional solenoids of the crane distributor. As the oil temperature increased - the pilot pressure decreased, resulting in insufficient spool travel. Therefore low speed (insufficient oil flow) was being caused by normal response of the pump load-sensing control to the constriction of oil flow in the distributor valve. The malfunction was discovered through measuring the delta P between the pump outlet and the LS line - which equaled the preset pressure differential value and was constant all the time, which also meant that the control part of the pump was working. Had the insufficient flow been caused by the pump's low efficiency, the delta P reading would fall below the control setting (due to flow reduction caused by the excessive leakage in the pump, and not by the control response), and also oil overheating would be present.

    Malfunction two - piece of equipment - an Atlas Copco sample drilling rig, powered by an electric motor driven Rexroth A10VO open circuit pump, equipped with a standard DFR (LS and max Pressure) control. The initial symptom, that triggered the assistance call, was excessive current consumption, which was causing periodic shut-downs of the electric board.

    Tests showed that the excessive power draw was being caused by the load-sensing regulator of the pump, which was set to an unusually high value (above 80 bar), and since the drill head motor didn't require full flow from the pump to operate at the desired speed, the unusually high delta P was causing a lot of energy go to waste across the spool of the distributor. Due to the fact that the rig was equipped with very efficient water coolers, and high flow of cold water was readily available at the site, oil overheating was not present. The pump was flow tested and showed very good efficiency and controllability. However when the delta P was set to a normal value (around 20 bar) and the pump was reconnected to the machine's distributor valve - something strange happened. Whenever the system pressure reached certain level, the flow would start to decrease. The current consumption, on the other hand, would stay almost at the same level - the hydraulic system behaved almost as if the pump had been equipped with a torque limiter, the only thing was - it wasn't! It also became clear why the load-sensing delta P setting had been set to such an unusually high level - earlier, when this initial symptom (the reduction of the drill head motor speed with the increase of pressure) emerged, the problem was "amended" by sky-rocketing the delta P (most probably the first adjustment screw the fiddler was lucky to fiddle with). Again, reading the pressure differential between the pump outlet and the LS signal line (near the pump control module) showed that a constant delta P of 20 bar was present all the time...

   As you can see - the two malfunctions have similarities. In both cases we have closed center load sensing systems, which use the same type of pump. In both cases the malfunction symptom is insufficient equipment speed, caused by insufficient flow. And in both cases we read a stable delta P between the pump outlet and the LS line (at the control module). It would seem that in the second case, just like in the first one, we would need to look for unwanted strangulation in the oil passage between the pump outlet and the place the LS signal "comes from" - and it was looked for and was not found, everything was OK - the hose and the pressure filter at the pump outlet were introducing very insignificant pressure drop, the distributor spool made full travel, and would maintain its position - yet the flow would steadily decrease with the pressure rise...

   Before going into the causes of this malfunction, let us consider the classic closed center load sensing pump control for a minute. Many folks think that it controls flow, but it actually ain't! Not directly anyway, classic load sensing control is nothing but a pressure control, which simply tries to keep the pump outlet pressure X bar (the delta P setting) above the signal pressure value, fed to the LS port. That's it! Simple math - feed the LS port with five bar, and you get five plus delta P at the outlet, feed it with a hundred - and you get a hundred plus delta P, or less, of course, if there's no pressure demand from the system, as in the end it is the actuator's resistance to oil flow that defines the pressure in the circuit, but - not more! Then we can use this quality to keep oil flow at a required level, by introducing a restriction (in this case a distributor spool), and feeding the LS port with the pressure signal taken after the restriction. Maintaining steady delta P across a fixed restriction equals stable flow, change the restriction - and the flow will also change, as the control will try to maintain the pressure differential. Of course, when the restriction is wide open, and the pump hits its maximum displacement - the delta P drops, but whenever the flow is being modulated by the control - the set pressure differential is present and constant.

    Let us look at our drilling rig - what is happening?
   - Drill motor speed steadily decreases as the system pressure rises.
    What causes what?
   - Low operation speed is caused by low oil flow.
   - Low oil flow is caused by the reduction of the pump displacement.
   - Reduction of the pump displacement is caused by the load sensing control of the pump, which is doing his job by keeping the stable delta P between the pump outlet and the LS signal (or, in other words adjusting the pump outlet pressure delta P bar above the signal pressure in the control's LS port).

    Therefore - since we already saw that the open distributor spool, the pressure filter and the pump hose introduce restriction that remains unchanged, the only explanation for what is happening is the increase of the pressure drop in the LS line between the distributor and the pump control, occurring with the rise of the system pressure. How can this be possible? The right question here would be - what can create a pressure drop that increases along with pressure? The answer is - oil flow through a choke point (obviously from the area of high pressure to the area of low pressure)! In this case it means that somewhere in the LS line there is an oil leak to tank, which results in a system pressure dependent oil flow, and the pressure drop in the line, created by this flow is adding to the pressure drop already existing between the pump outlet and the distributor LS signal port. When the sum of the two overcomes the preset delta P - the pump control starts cutting the displacement, as it should.

    The system was equipped with a remote pressure control valve, connected to the LS line. The cartridge seals were completely gone, which created a significant oil leak between the LS line and the drain - causing the above symptom. This schematics explains how this was happening. In the end the seals were replaced, and the rig drilled happily ever after.

      Final thoughts:

1. Load sensing pump control is a pressure control, which tries to adjust the pump outlet pressure x bar above the LS port pressure. 2. A good way to check load sensing systems is by reading delta P, which is, by the way, best performed by digital gauges, capable of doing the math for you - a great tool every technician should have and know how to use.
3. Flow reduction in a variable pump load sensing system can be caused by either a) increase of the flow restriction between the pump outlet and the place where the LS signal is taken from, or b) the flow present in the LS signal line, as the pressure drop inside the LS line will add to the delta P in the distributor.
4. Fiddlers are a nice kind of people... They have successfully provided us with steady flow of work since the beginning of the industrial hydraulics age...