The flow from hydraulic pumps can be fixed or variable depending on the type of pump being used. For fixed displacement pump systems the simplest form of flow control is obtained by generating a level of pump pressure that causes opera­tion of the relief valve but this method can be extremely inefficient. Alternative valve systems can be used to improve this situation particularly when operating several services from a single pump. The type of circuit used in this case will depend on whether the pump is fixed or variable diaplacement.