Hydraulic systems sometimes have a manual relief valve integrated into the design. This valve relieves built up pressure and is usually deployed when the mechanical system fails at some point and stops the pressure being relieved automatically according to the normal operation of the equipment. Essentially it is a safety feature that does not rely on a mechanical or electronic process but on human interaction which over-rides the system, returning the machinery back to an idle position. In cases where there is no manual relief valve (for example, sub-sea mining where equipment operates autonomously), a relief valve will still be in place but can be operated remotely.
The singer Katy Perry recently demonstrated the importance of a manual relief valve, when she became stranded on a stage prop during a show. Known for her elaborate performances and costumes, the enormous model of Saturn on which Perry got stuck, was just one of several large scale moving stage sets that are part of her Witness tour, which includes a giant mechanical hand which grabs her and pulls her under the stage. At the time of the incident Perry had finished a song and was expecting the planet prop to be lowered back down to the stage so she could disembark, but a malfunction meant that instead she was left stranded for a couple of minutes while stage hands rushed about to manually lower the equipment. While she waited Perry entertained the crowd, eventually diving into the audience when the prop was low enough.
Although highly embarrassing for Katy Perry, she was not in any danger during the ordeal but in medical applications where hydraulics are used to raise, lower and turn equipment there may be situations in which the patient’s safety is compromised by being stuck in a position that is not compatible with their condition. A manual relief valve allows for the equipment to be returned to the normal position quickly and without further involvement from staff. Dentist chairs operate on the same principle. If someone was to suffer a medical emergency while at the dentist, the emergency services would need to chair to be quickly returned to the right position in order to treat or move the patient.
Manual relief valves in hydraulic systems are not always used for personal safety; the build up of pressure in a hydraulic system generates heat which can damage the internal components of the machinery, including the hydraulic motor, the cylinders and the seals between parts of the system. If equipment malfunctions and the pressure cannot be released then the cost of repairing the machinery could have serious implications for the business in terms of outlay and in downtime, which doubles the loss of productivity and revenue. Being able to release the pressure in a system with one single over-ride valve means that the potential damage can be limited to a point where it may not have caused any serious problems and will not take too long to repair. On-site engineers can work on the machinery straight away, without having to first work out how to release built up pressure and this makes it much safer for them to operate on the hydraulic machinery as there is no risk of that pent-up energy suddenly being released in an unsafe manner.
If your hydraulic equipment does not have a manual relief valve there may well be a good reason for that – if the system has built in failure contingencies or is a simple system that is not prone to malfunction then there may be no need. However, a manual relief valve can be retrofitted in many systems and should be a consideration for any machinery where there is a risk to personal safety or to the integrity of the equipment.