Hydraulic Pump Rating and Selection Factor.

Pump Rating and Selection Factor.

The performance of different pumps is evaluated on the basis of many factors, including:

Physic characteristics, Operating Characteristics and Cost. When selecting a pump, the following pump rating and selection factors are considered:

            Capacity

Pressure

Drive Speed

Efficiency

Reliability

Fluid characteristics

Size and weight

Control adaptability

Life

Pump Installation and maintenance costs.

Some of the selection factors for different pumps are given in Table 5 – 1. Each of these selection factors is described briefly in the following paragraphs.

 
Capacity 

The primary rating of a pump is its capacity. This is also called the delivery rate, flow rate, or volumetric output. The capacity is usually given in gallons per minute, cubic inches per minute, or cubic inches per revolution at specified operating conditions. Pump capacity ratings are usually given at standard atmospheric inlet pressure and various output pressures, as well as at approximate fluid service temperatures.

Pressure

The pressure rating of a pump is generally based on the ability of the pump to withstand pressure without an undesirable increase in its internal leakage (or slippage) or damage to pump parts. Pumps are pressure – rated under the same conditions (speed, temperature, and inlet pressure) at which they are capacity rated. Most pumps are pressure rated at 100, 500, 1000, 1500, 2000, 3500, and 5000 psi (pound per square inch).

Drive Speed

Pumps are often rated at the commonly available electric motor speeds of 1200 or 1800 rpm. They may also be rated at speeds other than motor speeds. For instance, higher speeds occur in mobile hydraulic pumps driven from internal combustion engines. These engines usually operate at a constant speed and include speeds of 2000 rpm. And higher. Some industrial hydraulic pumps are rated at speeds up to 4000 rpm.

The maximum safe speed for a rotating pump is limited by the pump’s ability to avoid cavitations and high outlet pressures. Most pumps also require a minimum operating speed. Although these speeds are usually not critical, pumps operating at high pressures require a minimum speed to prevent overheating or internal slippage.

Maximum speed and pressure ratings for pumps are often given for both intermittent and continuous operation. Continuous ratings describe the maximum speed and pressure at which a pump can be operated for a normal design life (about 10000 hours). Intermittent ratings are the maximum speed and pressure at which a pump can be operated safely for short times and still have a satisfactory service life. Operating a hydraulic pump beyond any ratings will usually reduce its service life.

Efficiency

As pointed out earlier, the pressure a system exerts on the hydraulic pump directly affects the delivery rate of the pump. As the pressure increases, the flow rate of the pump decreases. The amount of decrease varies depending on the type of pump used. This change in flow affects the pump’s efficiency. Pump efficiency is described in two ways:

VOLUMETRIC EFFICIENCY–the ratio of actual delivery rate to the theoretical displacement.

OVERALL EFFICIENCY–the ratio of its hydraulic power output to its mechanical power input.

Hydraulic Pumps

Introduction

Previous lesson in this unit have explained what may have appeared to be unimportant parts of a hydraulic system. However, they are all really important if the system is to function properly, and this lesson describes one of the most important parts of the hydraulic system – the pump; and explains its operating characteristics.

Pumps are described according to their different types of construction, operating characteristics, and design features. From this lesson you will learn important maintenance and installation requirements of hydraulic pumps and boosters. When you know how pumps are constructed and how they operate you will be able to troubleshoot the system quickly and with confidence.

Pump Classification

Pumps are used in hydraulic system to convert mechanical energy into hydraulic energy. More specifically, a pump converts the kinetic energy of a rotating shaft into the kinetic energy of fluid flow. The fluid flow also has potential energy that allows it to overcome the resistance of the system to fluid flow. Remember that a pump produces flow, not pressure. Hydraulic pressure is caused by the resistance of the hydraulic system to fluid flow.

When operating, a hydraulic pump creates a partial vacuum at its inlet, permitting the atmospheric pressure in the fluid reservoir to push the hydraulic fluid through the inlet strainer and line into the pump. The pump then transfers this fluid to its outlet and into the hydraulic system as shown in Fig. 5 – 1. As the fluid leaves the pump it encounters the back pressure in the system. This back pressure is built up by the pressure regulating valve, the system work load, and also by flow losses in the hydraulic tubing.

Pumps are classified on the basis of the physical arrangement of their pumping mechanism and their basic principle of operation. Pumps classified on principles of operation include POSITIVE displacement and NON POSITIVE displacement types. Positive displacement pumps are equipped with a mechanical seal (gear, vanes, or impellers) between the inlet and outlet, which reduces internal leakage or slippage. Therefore, the output of positive displacement pumps is almost unaffected by variations in system pressure.

Non – positive displacement pumps do not have a positive internal seal against leakage or slippage. Because of this slippage, the delivery of these pumps is reduced as the back pressure of a system is increased. However, the non – positive displacement pumps deliver a continuous flow, while the positive displacement pumps deliver a pulsating flow. These pulsations are small and can be smoothed out by the accumulator or the system piping. Virtually all hydraulic system pumps are positive displacement pumps.

Positive displacement pumps have either a FIXED or VARIABLE DISPLACEMENT. The volume or gpm (gallon per minute) of a fixed displacement pump can be changed only by changing the speed of the pump, because the physical arrangement of the pumping mechanism cannot be changed.  (This does not mean that the flow in other portions of the system cannot be adjusted by valves)

The flow of a variable displacement pump can be changed by changing the physical arrangement of the pumping mechanism with a built – in controlling device. This device often functions in response to system back pressure or other signals. Variable displacement pumps are more complex than fixed displacement pumps and, therefore, cost more. In addition, the internal efficiency of a variable displacement pump is lower than that of fixed displacement pump. This is offset somewhat by the higher overall efficiency of a system powered by a variable displacement pump.

Most positive displacement pumps are classed as ROTARY pumps. This is because the assembly that transfers the fluid from the pump inlet to the outlet has a rotating motion. Rotary pumps are further classified according to the type of element, or part that transfers the fluid , such as gears, vanes or screw.

A different positive displacement pump is the PISTON pump. This pump uses a reciprocating or back – and – forth motion of the piston, alternately to trap fluid on the pump inlet side, and to discharge the fluid on the pump outlet side. A RADIAL PISTON pump has a revolving assembly with several piston assemblies built into it, and can be classified as a rotary pump. Several types of piston pumps will be discussed later in this lesson.

Types of Accumulator

The Diaphragm Accumulator

The Diaphragm accumulator consists of two dome – shaped shells held together by threaded or bolted flanges. The diaphragm is clamped between them as shown in Fig. 4–12. The diaphragm has an even thickness and is molded with a single convolution (wave or roll) to allow it to move more easily. During operation, the diaphragm flexes instead of stretching like the air bladder. It is not, therefore, suitable for high – pressure applications.

The Gas-Charged Piston Accumulator

The Gas – Charged Piston accumulator, shown in Fig. 4–13, can be compared to a diaphragm accumulator. It consists of a free float piston, placed in a cylinder, which separates the gas and fluid. The cylinder is precision machined, honed, and capped, and the piston is equipped with suitable seals to ensure positive separation of the fluid and gas. The end caps of the cylinder are retained by various means, including threading, lock rings, tie rods, and welding. The accumulator is safety – locked to prevent it from being disassembled while it is pressurized. Also, a safety – burst disc is sometimes built into the gas end to protect the accumulator from excessive pressures and to prevent serous accidents.  

Although the fluid capacity of some gas – charged piston accumulators may reach 20 gallons, the response of the piston accumulator is fairly fast. However, the gas – charged accumulator is less effective than the bladder accumulator in eliminating pulsations because of the mass of the piston and the friction of the seals. Disadvantages include cost, limitations in size, maintenance, and frequent recharging of the gas side. One of their advantages is that they are suitable for both high and low temperature operation when the proper O – ring seals are used.

The Differential Piston Accumulator

The Differential Piston accumulator is similar to the gas – charged piston accumulator. As shown in Fig. 4–14, it consists of a large – diameter air piston in an upper cylinder, which bears against a small – diameter fluid piston in a lower cylinder. This accumulator allows a smaller amount of air pressure to control a higher fluid pressure. It is also used as a pressure booster or fluid intensifier. 

The gain in pressure is offset by a smaller amount of fluid flow. The ratios of volume and pressure can be obtained by the following formula:

Ratio = Area of the air piston / Area of the fluid piston

For instance, if the gas piston has twice the area of the fluid piston, the fluid has twice the pressure of the gas and one-half of the actual volume.

Gas-charged accumulators depend on the compression of a gas for their fluid capacity and pressure level. If the gas temperature increases slightly, the gas volume also increases, and the fluid volume decreases. If the temperature of the gas cools, the opposite occurs. When selecting a differential piston accumulator, make sure that the selection is based on the volume of the fluid piston.

Types of accumulator

Gas Accumulator

The Air–Bottle or non separated, gas accumulator in Fig. 4–10 is a fully enclosed shell, mounted vertically, which has a fluid port on the bottom of the shell and a pneumatic charging valve on the top. When charged, air under pressure is confined in the upper portion of the shell by the fluid in the lower portion. There is no physical separation between the gas (air) and the fluid. These accumulators are usually fitted with some type of limit switch to control the fluid level and prevent gas from escaping out of the fluid port in the bottom. To prevent the gas from leaving the accumulator during high rates of fluid flow, about one – third of the fluid remains in the shell.

The fluid capacity of air – bottle accumulators is quite large because of their no mechanical design. The fluid capacity is only dependent on the size of the vessel used.  Although the response is fast, the major disadvantage of this accumulator is the fact that gas is absorbed or dissolved by the fluid under high pressures and can cause cavitations in the system. As a result, non separated gas accumulators should not be used with high speed pumps. Another disadvantage is that a separate compressor is usually required to charge and maintain the gas pressure in the vessel.

Bag Accumulator

Fig. 4–11 shows a Bladder or bag type accumulator. This accumulator is a seamless, high – pressure cylindrical shell that encloses a pear – shaped synthetic rubber bladder. The bladder is molded to an air stem, which contains a high – pressure air valve and is mounted with a high – pressure seal in the upper end of the shell. The bottom or fluid end of the shell is sealed by a special plug and spring – loaded poppet valve that allows fluid to flow in and out of the shell. The poppet valve is used to prevent the bladder from trying to flow out the fluid port. In addition the plug has a safety feature that prevents disassembling of the accumulator if there is any pressure in the system.

Before being put in operation, the accumulators is preloaded with air and then charged with oil from the system, thus compressing the air and the bladder. As the system needs fluid, the bladder expands; first at the top (where its diameter is largest and its wall thickness is least) and then, gradually, the bladder stretches downward and outward against the walls of the shell. This action gives the bladder its high volumetric efficiency and pressure range.

Even though the bladder has a fluid capacity of approximately 20 gallons, its response is as fast as the discharge valve will permit. The bladder also has very low inertia, making it a good pump pulsation dampener. In addition, it maintains a positive separation between the fluid and gas. This is especially important when considering the high pressure conditions for which their use is recommended.

Types of accumulator

Gravity accumulator

The Weight-Loaded or Gravity accumulator, shown in Fig. 4–8, consists of a long, finely ground and polished vertical steel cylinder that is fitted with a long, close fitting, smooth finished piston. A sealing device of some type is fitted into the cylinder wall to prevent fluid from leaking past the piston. Weights are mounted or placed on the piston to maintain a constant fluid pressure with in the cylinder and the remainder of the system. The amount of weight depends on the system pressure. The piston is prevented from over traveling by limit switches that turn the pump off when the level is too high and turn the pump on when the level becomes low.

The fluid capacity of most weight – loaded accumulators does not exceed 250 cubic inches (slightly over one gallon). Weight – loaded accumulators are used infrequently because they are large, heavy, costly, and sluggish. Their response to changes in fluid demand is slow especially during high input surges because of the large mass of the weights and the frictional drag of the pressure seals.

Spring – Loaded Piston accumulator

The Spring-Loaded Piston accumulator, shown in Fig. 4–9, is similar to the weight –loaded accumulator in design and principle. The pressure is maintained within the system by one or more springs. As excess system fluid is admitted to the cylinder, the piston raise, compressing the springs. As the system required fluid, the springs expand and force the piston down, adding fluid to the system. The amount of fluid in the cylinder and the system is controlled by pressure switch that turns the pump on when the system pressure reaches the minimum pressure point and turns it off when the accumulator is charged and maximum pressure is reached. Most spring – loaded accumulators have a safety stop switch that limits piston travel and also shuts off the pump if the spring breaks

Like weight – loaded accumulators, the fluid capacity of spring – loaded piston accumulators is small (usually less than a gallon). Although spring – loaded piston accumulators have a low maintenance cost, they are comparatively large and expensive for their capacity. Their response is fairly fast, but their use is limited to small volume and low pressure applications.