Pressure limiting valves are also known as check valves, non-return valves, pressure relief valves, pressure control valves. In motor vehicles, there are a total of four types of pressure-limiting valves, one is the fuel system pressure-limiting valves; the second is the lubrication system pressure-limiting valves, which are located in the location of some models installed in the oil pump, some models are installed in the oil filter, and some are installed in the main oil pipeline; the third is the water tank cover and the oil tank cover on the pressure-limiting valves; and fourth is the pressure-limiting valves of the hydraulic lift system.
The basic structure and working principle of all pressure-limiting valves are the same, only the installation location and working medium differ.
The basic structure and working principle of a pressure limiting valve.
Principle of operation of a pressure limiting valve
Most of the time, the pressure limiting valve is closed, only when the pressure of the working medium exceeds a certain value, the pressure limiting valve is opened, from the bypass discharge part of the medium, so that the pressure in the system is reduced; when the pressure of the system reaches the lower limit, the pressure limiting valve is closed, and the pressure of the system is raised again. So week after week adjust, so that the media pressure in the system is often at a reasonable level, to ensure that the fuel system, lubrication system, cooling system, and hydraulic system work properly.
The pressure limiting valve restricts the oil pressure in the circulating oil line to prevent excessive oil from entering the combustion chamber, increased oil consumption and abnormal engine operation. At the same time, to avoid excessive oil pressure, resulting in bursting of lines and filters. The pressure limiting valve is a one-way valve that controls the oil pressure through the pressure of a spring. When the oil pressure in the oil circuit is greater than the spring pressure, the valve ball opens and part of the oil flows back to the oil sump, depressurizing the oil circuit. It is usually mounted on the oil pump or main oil line.
The traditional fuel tank cap works similarly to a radiator cap with at least one pressure limiting valve, such as an air vapor valve used to balance the pressure inside and outside the tank. With the gradual consumption of fuel in the tank, the oil level continues to fall, forming a negative pressure above the oil level, when the vacuum reaches a certain level, the air valve (i.e., intake valve) of the fuel tank cap is pushed open by the atmospheric pressure, and the outside air enters the tank so that the pressure inside and outside of the tank reaches equilibrium.
If the air valve fails to open for any reason, resulting in significant negative pressure in the fuel tank, the fuel pump may not be able to draw fuel, resulting in an unstable engine idle or even sucking the fuel tank flat. When the temperature rises and there is too much fuel vapor in the fuel tank, and the fuel vapor pressure is higher than the atmospheric pressure, open the vapor valve in the fuel tank cap, and the fuel vapor (HC) in the fuel tank escapes from the valve into the atmosphere, so that the pressure inside and outside of the fuel tank reaches a balance and prevents the fuel tank from swelling and cracking.
Method for determining leakage from pressure limiting valves.
Several situations can occur with pressure-limiting valves
For the fuel system pressure limiting valve, if the following situations occur: the engine is difficult to start, the phenomenon of “lack of cylinder” after starting, and it stalls with a load; when using the hand pump to pump fuel, it feels like there is very little resistance; the amount of fuel supplied can not keep up with the amount of fuel, as if there is air, at this time, it should be checked to see if the injection pump pressure limiting valve is leaking.
For the lubrication system in the pressure limiting valve, the more intuitive failure phenomenon is the engine “burning tile”, sometimes replaced oil, and gaskets, cleaned the crankshaft and lubricating oil channel, a put it into the load running and burning tile. This phenomenon is often the oil pressure limiting valve leakage, resulting in low oil pressure, in the friction vice is difficult to form an effective lubricant film, thus causing burning tile.
For the hydraulic system’s pressure limiting valve, if the working device does not lift, only when the big throttle can lift, the small throttle will automatically sink, and sometimes you can hear the sound of oil leakage, indicating that the hydraulic system’s pressure limiting valve has a leakage fault.
For the vacuum decompression valve (essentially an intake valve) on the tank, a handheld vacuum pump can be used to test the valve: first, clean the vacuum decompression valve body, then connect a handheld vacuum pump to check the valve’s opening pressure and vacuum level, the opening pressure should be 15.3 to 20. 0 kPa and the vacuum level should be 6.0 to 3. 3 kPa if it exceeds the above ranges. The tank cover assembly should be replaced.
Common failure phenomena of pressure limiting valves.
There are several possibilities for pressure-limiting valve failure
- Valve body and seat wear
Most abnormal leaks in pressure-limiting valves are caused by wear, which is usually caused by unclean media. Pressure-limiting valve wear is characterized by the appearance of wear grooves or rust, resulting in a poor valve seal. In this case, the valve seat must be removed from the housing with the aid of a special tool and repaired. In case of slight wear, it can be placed on a platform and polished with fine sand. If the wear is severe, the part should be replaced with a new one.
When the new valve body is installed into the valve seat, it is recommended to hammer with a copper rod to make the valve body and the valve seat fit closely. When hammering, it should be noted that the copper rod must be aligned with the centerline of the valve seat, and the hammer drop point should be flat, and can not be offset. Then grind, and finally use kerosene penetration method to check whether the sealing performance meets the requirements, requiring at least 1 min without leakage.
- The pressure limiting valve is clogged, causing the pressure warning light to illuminate
At this point, the pressure limiting valve should be removed and thoroughly cleaned to completely remove any caked material from the valve.
- The pressure relief valve is stuck in the closed position, or the pressure limiting valve is matted up with debris and stuck in the open position
In either case, the parts of the pressure limiting valve (including the seat) should be thoroughly cleaned and replaced with new parts if necessary.
- The adjusting spring is deformed, broken, or weakened
If the spring force of the adjusting spring is weakened, a washer with a thickness of approx. 1 mm can be placed inside the spring seat to compensate for this as a temporary emergency measure and to restore the preload of the adjusting spring; if the adjusting spring is broken, the adjusting spring must be replaced with a new one.
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