Variable displacement pump circuits are called demand flow circuits. A simple example is shown in Fig. 4.6. (The astute reader will quickly observe that this circuit is the first circuit we have studied with no relief valve. Although no relief valve is shown, it is recommended that a relief valve always be used to ensure that pressure can never reach an unsafe level. Rapid deceleration of a large load can produce a dangerous pressure spike.) Before understanding how a demand flow circuit operates, it is necessary to learn about the operation of a particular variable displacement pump—the vane pump.