Whichever type of pump is used in a hydraulic system it will invariably be a ‘positive displacement’ pump, which means that the inlet and outlet ports are effectively sealed from one another (unlike in centrifugal pumps, which are typically used on central heating systems, for example). This means that if the pump shaft is rotating, then fluid must be being pushed out of the pump (Fig. 2.19). If that fluid has nowhere
OVER-PRESSURISATION RELIEF-VALVE PROTECTION
Fig. 2.19 Pressure-relief valve function
POINT OF INTEREST Due to space limitations, reservoirs on mobile machinery tend to be smaller than those on industrial systems. Special care has to be taken, therefore, to ensure that heat generation and air entrapment in the fluid are minimised.
WARNING
Where regular maintenance activities are required in a hydraulic system (e.g. topping up the reservoir fluid), the design of the system should be such that the correct way of carrying out the activity is also the easiest way.
to go, for example when a cylinder piston has reached the end of its stroke, the pressure will build up in the system until either something breaks or the drive motor stalls. Either occurrence is obviously not desirable, so to prevent an excessive build- up of pressure, a pressure-relief valve is nearly always a basic requirement in a hydraulic system.
In its simplest form, a pressure-relief valve consists of a seat, poppet and spring.
Usually there is an adjusting device for the spring so that the valve can be set at different pressures as required. This is referred to as a direct-acting relief valve (Fig. 2.20), because the pressure acts directly on the main poppet of the valve.
CLOSED
RELIEVING
Fig. 2.20 Direct-acting relief valve (Image courtesy of Eaton Corp.)
As long as the pressure on the inlet port of the valve is low, or less than the valve setting, the spring holds the poppet onto its seat and prevents flow through the valve. Eventually the pressure will rise high enough to overcome the spring force and push the poppet off its seat, allowing flow to pass through the valve and back to the reservoir. The valve will, therefore, limit the maximum pressure on its inlet port (and thus in the pressure line of the system) to the setting of the spring, protecting the system from over-pressurisation and potential damage. With very few exceptions, just about every hydraulic system using a positive-displacement pump will include at least one relief valve to limit the maximum system pressure. However, depending on the application, a direct-acting valve may not always be the most suitable choice.
In order to protect the whole system, the size of the poppet and seat of a direct- acting relief valve must be large enough to pass the full pump flow without undue restriction. The area of the poppet exposed to the inlet pressure will be the same as the seat area, so with large flow rates and high pressures a very stiff spring will be required to hold the valve closed up to the required pressure setting.
If the spring is very stiff there will be a significant difference in pressure between that required to start lifting the poppet off its seat (cracking pressure) and the pressure TOP TIP
The main system relief valve should be fitted in such a way that the pump outlet has direct access to it at all times.
required to open the valve fully (full-flow pressure). This pressure difference is known as the pressure override of the valve. However, having set up the valve to the required maximum system pressure with the valve passing the full pump flow, the valve could start to open and pass a small flow at the lower (cracking) pressure, thus creating heat and robbing the system of useful flow.
To overcome the pressure override problem, most main system relief valves use the pilot-operated (two-stage) principle. In this case the direct-acting valve acts as a pilot stage to control a main poppet. The main poppet incorporates a small orifice connecting the top and bottom, and is held onto its seat by a light spring (typically 2 bar (30 psi)). When the system pressure is below the setting of the pilot-stage valve, its poppet remains seated and no flow takes place through the main poppet orifice. The pressures on the top and the bottom of the main poppet will, therefore, be equal, but the light spring will hold the poppet onto its seat and the relief valve is closed (Fig. 2.21).
PILOT STAGE
MAIN STAGE
CLOSED
Fig. 2.21 Two-stage (pilot-operated) relief valve – closed (Image courtesy of Eaton Corp.)
When the system pressure reaches the setting of the pilot valve (Fig. 2.22), the poppet lifts off its seat and allows a small flow across it into the main tank port of the valve. This small pilot flow has to pass through the orifice in the main poppet, which therefore creates a pressure difference across it. When the pilot flow creates enough pressure difference to overcome the light spring (approximately 2 bar (30 psi)) the main poppet will also lift, thus relieving the full pump flow to tank. As opening the main poppet only involves compressing a very light spring, the pressure override will be much less than that of a direct-acting valve.
If the system pressure then drops, the pilot poppet will re-seat, blocking off the flow through the main poppet orifice. This will equalise the pressures again on the top and the bottom of the main poppet, and the light spring will push it back onto its seat and close off the main flow path through the valve.
Although pilot-operated relief valves are the most common type of relief valve used for the main system, some characteristics of the direct-acting valve may be
advantageous in other parts of the system. For example, direct-acting valves are very fast acting and so may be used where a relief valve is required to open very quickly to relieve a sudden peak pressure. Direct-acting valves are also very simple valves with very little that can go wrong with them. Two-stage relief valves, on the other hand, are more prone to fail in an open position due to, for example, an orifice blockage in the main poppet. In general, a relief valve failing open is safer than it failing closed, unless the valve is used to control a runaway load on an actuator, for example, in which case it would be undesirable.
Although relief valves provide very necessary protection in hydraulic systems, it must be appreciated that whenever flow passes across a relief valve at pressure, heat is generated. This is because the hydraulic power entering the valve (flow × high pressure) is greater than that leaving the valve (flow × low pressure), and the difference in power input and output is accounted for in the form of heat.
During machine idling periods, therefore, it would be very wasteful and require a lot of cooling capacity simply to allow a pump to pass its full flow across a relief valve at full pressure. To avoid this, many two-stage relief valves incorporate an extra port known as the vent connection, which is blocked, or plugged, for normal operation of the relief valve (Fig. 2.23). Opening the vent port to tank effectively collapses the pressure setting of the valve to a point where the inlet pressure needs only to overcome the light spring on the main poppet to open the valve. This then allows the pump flow to pass freely back to tank at very low pressure, thus avoiding significant heat generation when operation of the machine is not required.
In practice, this unloading function of a two-stage relief valve is often achieved by a small solenoid valve mounted on top of the relief-valve pilot stage. Normally the relief valve will be ‘vented’ (pump unloaded) when the solenoid is de-energised, and operate normally (full pressure setting) when the solenoid valve is energised (Fig. 2.24). This means that if there is an electrical failure of the solenoid valve, the system will revert to low pressure (‘fail-safe’).
PILOT STAGE
MAIN STAGE
RELIEVING
Fig. 2.22 Two-stage (pilot-operated) relief valve – relieving (Image courtesy of Eaton Corp.)
WARNING
Heat is generated whenever flow passes across a relief valve at pressure. A blowing relief valve is, therefore, likely to be very hot.
POINT OF INTEREST solenoid-vented relief valves are normally arranged such that in the event of an electrical power failure the valve is vented to low pressure (‘fail-safe’).