14.7 Light load protection
The temperature rise across a pump is caused by two separate phenomena; the isentropic compression temperature rise and the rise generated by the inefficiency of the pump. As throughput diminishes, efficiency falls and the temperature rises.
If a pump runs for any length of time without an adequate flow of water passing through it, overheating will occur which can lead to vapour locking. To prevent this, a leak-off system is provided which opens automatically at the minimum safe flow and allows the pump to discharge to the de-aerator. Pressure in the leak-off piping is broken down through a pressure reducing vessel, which consists of a series of orifice plates, and finally with an orifice plate positioned at the inlet to the de-aerator and sized to avoid flashing in the leak-off pipework.
Low throughput can also lead to random pressure pulsations being generated by the pump. These can excite low frequency resonances in the adjacent feed system pipework, leading to unacceptable pipework vibration. The minimum leak-off flow is therefore chosen to minimise the risk of these effects and, hence, to protect both the pump and the feed system. Experience with high speed feed pumps indicates that the leak-off quantity should be set to at least 25% of pump best efficiency flow to avoid any pipework vibration problems.
The configuration commonly used on CEGB stations (Fig 4.62) consists of 2 x 100% parallel slide leak-off valves, arranged in parallel, with a separate pressure reducing vessel downstream of the valves. The initiation signal for leak-off valve operation is taken from an orifice plate located in either the suction or discharge pipework. Installation in the suction pipework is preferred since this location senses total pump throughput, i.e., the summation of forward flow to the boiler and the leak-off flow. Also, high pressure differential tappings are not required in this position.
Electrically-operated valve actuators have been used on many stations but as failure of the electrical supplies would render them inoperative and pump protection would be lost, the electrical supply for each valve is taken from separate station switchboards to improve security. In recent years, improvements in electropneumatic systems have resulted in a preference for this fail-safe method of actuation, where the valves open on loss of air supply. A further advantage of the pneumatic system is that the speed of operation is readily adjustable.
To provide maximum protection to the pump and piping system under all foreseen operating conditions, the valves must be capable of opening rapidly. On the other hand, very fast opening can lead to unacceptable shock loadings on the feed system. From experience, valve opening times of up to 15 s have been found acceptable, dependent on the feed system layout. Consequently, valve actuators need to include a facility for varying and resetting the opening/closing times after installation, to suit local conditions and operating practices.
To permit on-load maintenance of the leak-off system, sufficient manual isolating valves are included and any parts subject to in-service wear, for example, leak-off valve seats, are designed to be readily replaceable without cutting or rewelding the pipework. All isolating valves should have a locking facility to avoid malfunction.