Hydraulic Pump

Vane Pumps

The pump shown in Figure 3-9 is referred to as an unbalanced pump because all of the pumping action takes place on one side of the rotor. This causes a side load on the rotor. Some vane pumps are constructed with an elliptical housing that forms two separate pumping areas on opposite sides of the rotor. This cancels the side load. This type of pump is referred to as a balanced vane.

Usually, vane pumps are fixed-displacement and pump only in one direction. There are some designs of vane pumps that provide variable flow. Vane pumps are generally restricted to service where pressure demand does not exceed 2,000 psi. Wear rates, vibration, and noise level increase rapidly in vane pumps as pressure demands exceed 2,000 psi.

2.3.1.3 Hydraulic Pump Design

In a hydraulic pump, the displaced volume is controlled by the inclined angle of the swash plate. The swash plate angle is controlled by the discharge pressure through the compensator valve and stroking piston.

Figure 2.22 shows the stroking piston nearly fully extended, putting the swash plate at a very slight angle and generating small output flow. The high-pressure discharge fluid connects to a compensator valve spool. If the user requires a high flow rate, then the discharge pressure decreases and the spring forces the spool to move to the right position. When the fluid pressure reaches the value defined by the setting of the spring, the spool valve transfers the fluid from the stroking piston to the case pressure. Increasing the swash plate angle will increase the output flow rate. The compensator valve is the integration from the volume of fluid to the stroking piston and pump output. If the flow demand drops and the discharge pressure rises above the compensated value, then the valve passes fluid to extend the stroking piston and the swash plate angle decreases to lower the discharge pressure.

11.3.8 Hydraulic Pumps

Hydraulic pumps convert electrical energy into fluid pressure by using an electric motor to drive the pump. They are necessary for all hydraulic drives. The fluid pressure is then delivered by hydraulic fluid to cylinders and actuators and hydraulic motors at the required pressure level and volume. Hydraulic pumps generally operate at higher speeds and pressures than hydraulic motors. While some drive systems use reversible pumps, most gate drives use a unidirectional pump with a directional control valve to reverse the operation of the actuators. Like for hydraulic motors, there are three basic types of hydraulic pumps including gear, piston, and vane. Within USACE, it is common and recommended [1] to provide redundant hydraulic pumps and electric drive motors. Each pump is identical and sized for the required loads to drive the hydraulic system. The pumps are cycled with every gate operation. Today, a majority of USACE gate drives utilize a design where the hydraulic pump and electric motor are installed as part of an HPU package (Fig. 11.23).

Pumps are of either fixed or positive displacement type or variable type. For fixed displacement pumps, the volume is controlled by the capacity of the pump and the speed of the electric motor. Fixed displacement pumps include internal and external gear pumps, axial and radial piston pumps, screw pumps, and vane pumps. Variable volume pumps are designed and constructed as variable flow or displacement and these are typically vane pumps.

The simplest and most rugged positive displacement pump, having just two moving parts, is the gear pump. Gear pumps have a high tolerance for fluid contamination, good overall efficiency, and are relatively quiet. Where fluid contamination is a continuing concern, gear pumps are likely a better choice. While these pumps are fixed volume at a given speed (in rpm), their flow rate and speed characteristics are linear within their efficiency ranges. Speed and direction control of a drive system can be provided by driving a reversible gear pump with a variable speed electric motor, motor. This is ideal for integral HPUs. Gear pumps generally are restricted to less than 24 MPa (3500 psi) service. The drive shown in Fig. 11.23 for the LPV 149 sector gate utilizes a gear pump. It is driven at 1765 rpm with a 5.6 kW electric motor.

The piston pump is also commonly used in gate drives. It has the highest volumetric efficiency, highest overall efficiency, highest output pressures, and longest life expectancy. This type of pump is available in variable displacement types with a large variety of control systems for pressure and capacity. The electric drive motor is often restricted to 900–1200 rpm in order to reduce noise and increase pump life. Piston pumps generally are restricted to less than 42 MPa (6000 psi) service which is more than adequate for the majority of gate drives. Axial piston pumps are used for high-pressure and high-volume applications and the pistons are arranged parallel to the drive shaft. The two basic types of axial piston pump are the swash plate and the bent axis designs. The bent axis design is typically considered to have less noise, vibration, and wear than the swash plate design. Swash plate pumps can be designed to drive a separate pilot pressure pump from a shaft extension, while bent axis pumps will require a separate electric motor and pump arrangement for pilot pressure.

Radial rolling piston pumps are both extremely reliable and a simple design. The pistons extend in a radial direction around a drive shaft. A typical design includes solenoid controls for up to five discreet operating speeds. Each of the operating speeds has a variable adjustment range from zero to full volume capacity to permit field adaptation to operating conditions. The typical pumping system includes an integral pilot pump, internal pressure relief valves, and associated control devices for speed of shifting between pumping rates.

Variable volume vane pumps are efficient and durable, as long as a clean hydraulic system is maintained. In a simple circuit, the pressure compensation feature of the vane pump reduces the need for relief valves, unloading valves, or bypass valves. Vane pumps generally are restricted to less than 14 MPa (2000 psi) service, however.

Waterbury Pump and Servo Valve

The hydraulic pump is driven at a fixed speed. The direction and quantity of flow of oil to either the tilt or rigging cylinders is determined by the position of the hydraulic pump's tilt box and this in turn is governed by the rotary servo valve. A diagram of this type of valve is shown in Fig. 7.11 where it is accompanied by a full description. The pilot valve controls the position of the pump tilt box by means of two hydraulically operated pistons, one having twice the operating area of the other. The outer rotor of the rotary pilot valve is coupled to the tilt box; the inner rotor is coupled to the shaft which receives its motion from the stroke control servo.

4.12.4.4.2.5 Pressure transducer system

It is often called a pressure transmitter, which is a transducer that converts pressure into an analog electrical signal. The conversion of pressure into an electrical signal is achieved by the physical deformation of strain gages, which are bonded into the diaphragm of the pressure transducer and wired into a wheat stone bridge configuration. Pressure applied to the pressure transducer produces a deflection of the diaphragm, which introduces strain to the gages. The strain will produce an electrical resistance change proportional to the pressure.

Publisher Summary

A hydraulic pump takes oil from a tank and delivers it to the rest of the hydraulic circuit. It raises oil pressure to the required level. This chapter illustrates the operation of such a pump. There are two types of pump (for fluids) or compressor (for gases). There are essentially three different types of positive displacement pump used in hydraulic systems. The difference in pressure between outlet and inlet ports creates a severe load on the vanes and a large side load on the rotor shaft, which can lead to bearing failure. Allowing excess fluid from a pump to return to the tank by a pressure relief valve is wasteful of energy and can lead to a rapid rise in temperature of the fluid as the wasted energy is converted to heat. It is normally undesirable to start and stop the pump to match load requirements, as this causes shock loads to pump, motor, and couplings.

1. Pump Suction Strainer

Cavitation of the hydraulic pump is the most frequent mode of hydraulic system failure. The most frequent source of cavitation is suction flow restrictions caused by dirt buildup on the suction strainer. This can happen on both new and older systems. It produces the symptoms just described: increased pump noise and loss of high pressure and/or speed.

If the strainer is not located in the pump suction line, it will be found immersed below the oil level in the reservoir. Some operators of hydraulic equipment never give the equipment any attention or maintenance until it fails. Under these conditions, sooner or later, the suction strainer will probably become sufficiently restricted to cause a breakdown of the whole system and damage to the pump.

The suction strainer should be removed for inspection and should be cleaned before reinstallation. Wire mesh strainers can best be cleaned with an air hose, blowing from the inside out. They can also be washed in a solvent that is compatible with the reservoir fluid. Kerosene may be used for strainers operating in petroleum-based hydraulic oil. Do not use gasoline or other explosive or flammable solvents. The strainer should be cleaned even though it may not appear to be dirty. Some clogging materials cannot be seen except by close inspection. If there are holes in the mesh or if there is mechanical damage, the strainer should be replaced.

When reinstalling the strainer, inspect all joints for possible air leaks, particularly at union joints. There must be no air leaks in the suction line. Check reservoir oil level to be sure it covers the top of the strainer by at least 3 inches at minimum oil level, which is with all cylinders extended. If it does not cover to this depth, there is danger of a vortex forming that may allow air to enter the system when the pump is running.

For application to water hydraulic pumps (axial piston type), influence of material combination and micro shapes of sliding surfaces on friction and seizure were experimentally investigated. Seizure tests under water lubricating condition were carried out by an end-face sliding test apparatus, and sliding pairs containing diamond-like carbon (DLC) coating which is considered one of hopeful materials by its excellent tribological property in water were prepared for the seizure tests. As a result of the seizure tests it is obtained that in combination of stainless steel and DLC coating critical contact pressure on occurrence of seizure increases with increase of radius of pad's edge of a stationary specimen. On the other hand, a result of the tests shows that combination of aluminium bronze and DLC coating has much higher critical contact pressure on occurrence of seizure than combination of stainless steel and DLC when micro shapes of sliding surfaces are similar. For application of DLC coatings to valve plates in water hydraulic pumps it is better to choose copper alloys as mating surfaces of DLC coating and it is required to round edges of pads on valve plates adequately.

5.3.3 High speed

7.1 Introduction

Suffice it to say that hydraulic pumps are a form of artificial lift, that is, the well no longer has sufficient energy to produce itself the required volume, and an additional energy must be supplied to compensate the energy deficiency. More specifically, hydraulic piston pumps are used for low-production volumes (approximately 0–200 bpd), while jet pumps can be used for volumes that range from very low to very high (12–35,000+ bpd).

There are just four essential components of a hydraulic system and from which a large number of variations have been derived.

The preferred style of hydraulic pump installations is known as a free style pump which means the pump is “free” to move down with the power fluid, that is, down to be installed and up to be retrieved. This eliminates the need to use a rig to install/recover the pump.

The downhole arrangement used for hydraulic pumps is determined by the production requirements of the well. For gas wells, the requirement is to remove the liquids in the wellbore that are creating a back pressure on the reservoir thereby inhibiting the inflow of gas. The liquid volumes are typically low, so an installation with parallel tubing stings is often used.