After reading this chapter, one will be able to:
• Know the various accessories used in a hydraulic system
• Understand the function and construction of a reservoir
• Understand various types of accumulators from their design and construction point of view
• Select and specify accumulators for various applications
• Understand the concept of heat exchangers and their functions
• Know the specifications and construction of pipes and hoses
• Select and size hoses and pipes for different hydraulics applications.
When we talk about hydraulics, it is not only fluids that come into one’s mind. Also a discussion on hydraulics is not complete with only a discussion on pumps, motors and valves. There are other important components and aggregates in a hydraulic system listed under the category of hydraulic accessories. These accessories provide a clean and uninterrupted supply of fluid to a hydraulic system. In this chapter let us concentrate on learning how these accessories contribute not only to an efficient but also effective hydraulic system.
The reservoir system
The ‘reservoir’ as the name suggests, is a tank which provides uninterrupted supply of fluid to the system, by storing the required quantity of fluid. The hydraulic fluid is considered the most important component in a hydraulic system or in other words its very heart. Since the reservoir holds the hydraulic fluid, its design is considered quite critical. The reservoir in addition to storing the hydraulic fluid, performs various other important functions such as dissipating heat through its walls, conditioning of the fluid by helping settle the contaminants, aiding in the escape of air and providing mounting support for the pump and various other components. These are discussed in detail below. Some of the essential features of any good reservoir include components such as:
• Baffle plate for preventing the return fluid from entering the pump inlet
• Inspection cover for maintenance access
• Filter breather for air exchange
• Protected filler opening
• Level indicator for monitoring the fluid level
• Connections for suction, discharge and drain lines.
The proper design of a suitable reservoir for a hydraulic system is essential to the overall performance and life of the individual components. It also becomes the principle location where the fluid can be conditioned in order to enhance its suitability. Sludge, water and foreign matter such as metal chips have a tendency to settle in the stored fluid while the entrained air picked up by the oil is allowed to escape in the reservoir. This makes the construction and design of hydraulic reservoirs all the more crucial.
Many factors are taken into consideration when selecting the size and configuration of a hydraulic reservoir. The volume of the fluid in a tank varies according to the temperature and state of the actuators in the system. The volume of fluid in the reservoir is at a minimum with all cylinders extended and a maximum at high temperatures with all cylinders retracted. Normally a reservoir is designed to hold about three to four times the volume of the fluid taken by the system every minute. A substantial space above the fluid in the reservoir must be included to allow volume change, venting of any entrapped air and to prevent any froth on the surface from spilling out.
A properly designed reservoir can also help in dissipating the heat from the fluid. In order to obtain maximum cooling, the fluid is forced to follow the walls of the tank from the return line. This is normally accomplished by providing a baffle plate in the centerline.
The level of fluid in a reservoir is critical. If the level is too low, there is a possibility of air getting entrapped in the reservoir outlet pipeline going to the pump suction. This may lead to cavitation of the pump resulting in pump damage.
The monitoring of the temperature of the fluid in the reservoir is also important. At the very least, a simple visual thermometer whose ideal temperature range is around 45 °C (113 °F) to 50 °C (122 °F), needs to be provided on the reservoir.
There are basically two types of reservoirs:
1. Non-pressurized reservoir
2. Pressurized reservoir.
As the name suggests this type of reservoir is not pressurized, which means, the pressure in the reservoir will at no point of time rise above that of atmospheric pressure. Very extensively used in hydraulic systems, these reservoirs are provided with a vent to ensure that the pressure within, does not rise above the atmospheric value.
Figure 7.1 shows the typical construction of such a reservoir conforming to industry standards.
These reservoirs are constructed with welded steel plates. The inside surfaces are painted with a sealer, to prevent the formation of rust which might in tum occur due to the presence of condensed moisture. The bottom plate is sloping and contains a drain plug at its lowest point, to allow complete draining of the tank when required. In order to access all the internals for maintenance, removable covers are provided. A level indicator which is an important part of the reservoir, is also incorporated. This allows one to see the actual level of the fluid in the reservoir, while the system is in operation. A vented breather cap with an air filter screen helps in venting the entrapped air easily. The breather cap allows the tank to breathe when the fluid level undergoes changes in tune with the system demand.
The height of the baffle plate in the reservoir is about 70% of the maximum fluid height. The purpose of the baffle plate is to separate the pump inlet line from the return line. This is done to prevent the same fluid from circulating continuously within the tank. In this way it is ensured that all the fluid is uniformly used by the system.
Essentially the baffle plate performs the following functions:
• It permits foreign substances to settle at the bottom
• It allows entrained air to escape from the fluid
• It prevents localized turbulence in the reservoir
• It promotes heat dissipation from the reservoir walls.
The reservoir is designed and constructed to facilitate the installation of a pump and motor on its top surface. A smooth machined surface of adequate strength is provided to support and maintain the alignment of the two units.
The return line enters the reservoir from the side of the baffle plate, which is opposite to the pump suction line. It should be below the fluid surface level all the time, in order to prevent foaming of the fluid. Similarly, the strainer or the foot valve should be located well below the normal fluid level in the reservoir and at least 1 in. or 2.5 ems above the bottom of the reservoir. If the strainer is located too high, it will lead to the formation of a vortex or crater that will permit ingress of air into the pump suction line.
The sizing of the reservoir is based on the following criteria:
• It should have sufficient volume and space to allow the dirt and metal chips to settle and the air to escape freely.
• It should be capable of holding all the fluid that might be drained from the system.
• It should be able to maintain the fluid level high enough to prevent air escaping into the pump suction line.
• The surface area of the reservoir should be large enough to dissipate the heat generated by the system.
• It should have sufficient free board over the fluid surface to allow thermal expansion of the fluid.
For most hydraulic systems, a reservoir having a capacity of three times the volumetric flow rate of the pump has been found to be adequate.
Although it has been observed that non-pressurized reservoirs are the most suitable ones in a hydraulic system, certain hydraulic systems need to have pressurized reservoirs due to the nature of their application. For example, the Navy’s aircraft and missile hydraulic systems essentially need pressurized reservoirs in order to provide a positive flow of fluid at higher altitudes where lower temperatures and pressure conditions are encountered.
The required pressure in the reservoir is maintained by means of compressed air. Compressed air is generally introduced into the reservoir from the top at a pressure specified by the manufacturer. In order to control this pressure, a pressure control device such as a pressure regulator is provided in the airline entering the reservoir. The function of this pressure regulator is to maintain a constant pressure in the reservoir, irrespective of the level and temperature of fluid in the reservoir.
A pressurized reservoir will only have a single entry point for filling up the fluid in the tank. Since the reservoir is always maintained at a pressure, it becomes important to have a foolproof system with safety relief valves, for the filling of fluid in the reservoir. Sufficient guidelines are provided by all manufacturers of such pressurized reservoirs.
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