Hydraulic power is an efficient method of delivering HP by pumping a fluid through a closed system. If the amount of flow or the pressure unknowingly decreases, the amount of HP delivered to a working unit will be reduced, and a system will not perform as it should.
Testers. Portable hydraulic-circuit testers (Figure 2-20) are lightweight units you can use to check or troubleshoot a hydraulic-powered system on the job or in a maintenance shop. Connect a tester into a system’s circuit to determine its efficiency. Currently, several hydraulic-circuit testers are on the market. Operating procedures may vary on different testers. Therefore, you must follow the operating directions furnished with a tester to check or troubleshoot a circuit accurately.
Improper Operation. When a hydraulic system does not operate properly, the trouble could be one of the following:
• The pump that propels the fluid may be slipping because of a worn or an improperly set spring in the relief valve.
• The fluid may be leaking around the control valves or past the cylinder packing.
Since hydraulic systems are confined, it is difficult to identify which component in a system is not working properly. Measure the flow, pressure, and temperature of a liquid at given points in a system to isolate the malfunctioning unit. If this does not work, take the system apart and check each unit for worn parts or bad packing. This type of inspection can be costly from the standpoint of maintenance time and downtime of the power system.
A flow control valve is a device used for adjusting or manipulating the flow rate of a liquid or a gas in a pipeline. The valve contains a flow passage or a port whose flow area can be varied. The role of a flow control valve in a hydraulic circuit is very important and its very location is critical to optimum system performance.
The basic function of a flow control valve is to reduce the rate of flow in its leg of a hydraulic circuit. One of the most important applications of flow control valves in hydraulic systems is in controlling the flow rate to cylinders and motors to regulate their speeds. Any reduction in flow will in turn, result in a speed reduction at the actuator. There are many different designs of valves used for controlling flow. Many of these designs have been developed to meet specific needs.
Some factors, which should be considered during the design stage of a flow control valve are:
• The maximum and minimum flow rates and the fluid density, which affect the size of the valve
• The corrosive property of the fluid, which determines the material of construction of the valve
• The pressure drop required across the valve
• The allowable leakage limit across the valve in its closed position
• The maximum amount of noise from the valve that can be tolerated
• The means of connecting the valve to the process i.e. screwed, flanged or butt welded.
Flow control valves are classified as:
Fixed or non-adjustable flow control valves represented symbolically as in Figures 6.28(aHd).
Adjustable flow control valves represented in hydraulic circuits as
Additionally they may also be classified as:
and pressure-compensated flow control valves represented as: