Hydraulic Problem?

Question

Details of the Hydraulic system
1) Gear Pump delivers a flow of 4Ltrs/min
2) The system pressure is 100bar (direct pressure relief vave is used)
3) Hydraulic oil 68 is the oil medium.
4) The hydraulic system is used to actuate 2 vertical pistons (cylinder) simultaneously.
5) The pressure ports are blocked when the gear pump is started.

Problem:
1) Oil gets heated to 48°C within 20mins of operation.
2) As the oil gets heated the pressure drops from 100bar to around 40bar. In this case the pistons cannot be moved upward (due to the load-weight on the piston).
3) The relief valve is the hottest component in the system.
4) After the system is switched off for 30mins. The pressure comes back to the normal 100bar and then again after 20 mins of operation the sytem pressure drops again...and so on
1) Is gear pump the problem
2) Is the relief valve the problem.
3) Can the problem be due to suction strainers
4) Is the Hydraulic oil 68grade the problem

Please help

Answer 1

Unwanted heat is a problem for all hydraulic systems. Even a well-designed system operating at top efficiency converts about 20% of its input power to heat. An inefficient system, or one poorly matched to its task, may convert nearly 100% of input power to heat at certain times in the cycle. The heat input can be dissipated through natural cooling; if this is insufficient, a heat exchanger is added to the system.

Liquid-to-liquid exchangers draw heat from the hydraulic fluid and transmit it into a cooling fluid, usually water. Most liquid-to-liquid exchangers use a shell-and-tube package, consisting of a bundle of small tubes inserted into a shell. The coolant flows through the small tubes, and the hydraulic fluid passes around and between the tubes. These units are compact, reliable, and are often less expensive to install and maintain than other types.

There are two basic types of shell-and-tube exchangers: the U-tube (or hairpin) type and the straight-tube type. Either type can have either a fixed or removable tube bundle. Removable bundles can be withdrawn from the shell as an assembly for maintenance, but fixed bundles must remain in the shell. A normal rule of thumb is that U-tube exchangers are best suited for high-temperature, high-pressure applications, with the straight-tube units most thermally efficient and least expensive.

Liquid-to-liquid exchangers are available in single or multiple-pass, parallel, or reverse-flow arrangements. The multiple-pass, reverse-flow units provide the greatest heat transfer for a given size. Standard liquid-to-liquid units have a working pressure of 150 psi and can handle temperatures to 300°F, although the actual temperature difference between oil and water should not exceed 200°F. Special units are available to operate at pressures to 300 psi.

Liquid-to-air heat exchangers transfer heat from hydraulic fluid to ambient air. Working much like an automobile radiator, they allow air to be passed over finned tubes containing the hot liquid. The finned tubes can be made of aluminum, copper, steel, or stainless steel, and are brazed or roller expanded to the header tank. Air is moved through the core by forced or induced-draft fans.

Air-cooled exchangers are most commonly used where water is costly or unavailable in sufficient quantities to dissipate the required heat, or where a portable heat exchanger is required. In some instances, they have been used to help supply plant heating requirements during winter months. These liquid-to-air heat exchangers are available in sizes to 100 hp, operating at pressures to 300 psi. Units up to 600 hp are available on special order.

Typically, liquid-to-air exchangers are larger, heavier, and noisier than liquid-to-liquid units. In return, they operate without necessity for water and they are portable. They require ambient air at least 10 to 15°F below the required oil output temperature for efficient operation. The only requirement for long life is that fins must be protected from clogging and dirty environments; a single mesh (window screen) overlay avoids fin clogging and provides for easy cleaning.

Location: All heat exchangers should be installed in the low-pressure side of a hydraulic circuit. This location eliminates the need for a high-pressure unit, which may be comparatively expensive. Heat exchangers should be protected against damage from high-pressure surges by a relief valve.

For large hydraulic systems operating at high pressures, a separate cooling circuit from a reservoir to the heat exchanger may be used to circulate the oil independent of changing flows in the main circuit.

For systems that are to be used outside, a system bypass line should be provided around the heat exchanger. Such a bypass line permits efficient year-round operation; the heat exchanger can be bypassed during cold weather starting until fluid has reached the operating temperature. Such a bypass line also permits maintenance of the heat exchanger without shutdown of the hydraulic system.

Experts recommend that line filters be installed upstream of the unit to protect the exchanger from excessive accumulations of dirt and scale, which can degrade thermal efficiency.

Protection: Generally, the colder the chilling fluid (air or water), the more heat will be removed from the oil -- up to a point. At temperatures below a certain level, the fluid may be too cold for efficient operation of the exchanger. Most hydraulic fluids tend to form viscous layers on contact with an extremely cold surface, and this stagnant fluid can create a thermal barrier within the heat exchanger. The colder the chilling fluid, the thicker is the viscous layer in the hydraulic fluid. For example, a 1-in. layer of noncirculating fluid has the insulating quality of a ¼ -in.-thick layer of rock-wool insulation. Therefore, the heat-transfer capability of an exchanger under cold-weather operation may be improved by restricting the temperature or supply of the chilling fluid.

Clean, soft water should be used in water-type heat exchangers to prevent corrosion and scaling in the tube bundle. If the only available water is hard (with excessive minerals) or brackish (with excessive salt), scale and dirt deposits can form in the small-diameter tubes. These deposits cut heat efficiency. Where fouling is possible, low cooling-fluid velocities should be avoided.

Another potential hazard to cooling efficiency is tube corrosion. Corrosion restricts coolant flow and can eventually perforate the tubes, permitting water to pollute the hydraulic fluid. Experts recommend use of a zinc anode or chemical inhibitor in the cooling water circuit to prevent or reduce this deterioration in the tube bundle.

Doubling up: If the flow requirements of the hydraulic system exceed the capacity of standard heat exchangers, two smaller standard units of equal capacity can be connected in parallel to dissipate the heating load. In this arrangement, the thermal load is shared equally by each small exchanger at a total system cost much less than a single large unit custom built for a system.

Many hydraulic circuits generate high thermal loads for relatively short periods. During normal operation, normal loads can be carried away by a standard liquid-air exchanger but, under extreme operating conditions, a small liquid-to-liquid unit is required to carry the additional load. In such circuits, the liquid-to-liquid exchanger is installed in series with, but downstream of, the liquid-to-air unit. With a bypass line around it, the water-type exchanger is held in standby during normal system operation. Again, this "ganged" arrangement is simpler and more efficient than use of a single, very large exchanger capable of handling peak loads.

Answer 2

The oil in the reservoir doesn't get hot, just a bit warm (48 degrees). I don't think this warming of the oil is enought to cause anything to malfunction.

You've said the relief valve gets hot (not sure how hot). If it does the relief valve may fail to seat and the system could loose pressure causing the faults you describe.

I would say a faulty pressure relief valve which fails to seat when hot is the most likely cause of the problem. It's not a certainty and more testing may be required.

I'm not familiar with 68 grade hydraulic oil - but it does only get warm. I expect any hydraulic oil could handle 48 degrees.

It's possible that the gear pump stops working properly when it gets warm - but it's really not that warm.

Suction strainers actually work better when the oil is warm - so it wouldn't be them.

The art of fault finding is something that some tradesmen are good at and others arn't causing a lot of unnecessary work when they get it wrong.

Answer 3

It seems that, with the relief valve getting so hot, it's due to friction caused by an un-seated valve and, if the relief system returns oil to the suction side, the suction temperature is increased thus increasing the discharge temperature more and an ever-increasing cycle of temperature increase is taking place which is drastically decreasing the oil viscosity.
So, yes, I think the relief valve is the problem.
On top of this, pressure loss due to 'Slip' (backflow through the pump itself due to reduced viscosity), will not help the situation.
One other thing is, that the pump rotors are badly worn and some reverse flow to suction is taking place.

Answer 4

Sounds like your system is pressurizing the oil, but only recirculating a small portion of the flow. This is causing the oil to heat up, which lowers the viscosity of the oil and lowers the pressure produced by the pump. To solve the problem, if this pump must run constantly in stand-by mode, you should increase the relief system size and consider the installation of a by-pass cooler to help control the temperature of the fluid.