Why we close discharge valve of centrifugal pump before starting?

Reasons for closing the discharge valve

The starting current can be reduced by closing the discharge valve. The current will be very high when any motor starts. When the pump is started with an open discharge valve, the discharge head will have greater resistance to the pump.

What will happen if discharge valve of centrifugal pump is closed?

If the discharge valve is closed and there is no other flow path available to the pump, the impeller will churn the same volume of water as it rotates in the pump casing. This will increase the temperature of the liquid (due to friction) in the pump casing to the point that it will flash to vapor.

What must you do before starting a centrifugal pump?

Every centrifugal pump must be primed, vented and full of liquid before it is started. The pump you are about to start must be properly vented and primed. The suction valve must be fully open.

When start centrifugal cargo pump the discharge valve of it should be open?

Precautions while starting a centrifugal pump:

The pump should be started against a closed discharge valve and as the pump RPM increases the discharge pressure also increases and when the discharge pressure is about 4 – 5 kg /cm², the discharge valve is slowly opened.

When starting a centrifugal pump the delivery valve is kept?

Detailed Solution. The delivery valve connects the pump outlet and the delivery pipe. It remains closed before the pump is switched on. When the pump builds up its pressure, it is opened and can be used to control or vary the discharge.

why we close discharge valve before starting the centrifugal pump

Why centrifugal pump has no relief valve?

Because maximum pressure with the discharge closed is only moderately above working pressure, a relief valve is not necessary for a centrifugal pump.

Should the discharge valve be opened before starting a positive displacement pump?

Precautions Prior to Starting Pump

Verify that all suction and discharge valves are open before starting the pump. Prime the pump and jog the motor to check the rotation.

Why should the position open or closed of all valves be checked before starting the pump?

Starting Against an Open Valve

Care should be taken to not begin opening the discharge valve too soon, as this would cause excessive reverse flow through the pump and require the motor to start under adverse reverse speed conditions.

How do you shutdown a centrifugal pump?

Does closing a valve increase pressure?

As the valve closes, the flowing liquid is forced to stop, resulting in a transfer of kinetic energy to potential energy, which ultimately causes a pressure increase.

What would happen if a positive displacement pump is started against a closed discharge valve?

A positive displacement pump should not operate against a closed valve on the discharge side of the pump because it has no shutoff head like centrifugal pumps do; a positive displacement pump operating against a closed discharge valve continues to produce flow, and the pressure in the discharge line increases until the ...

What is shut off head in centrifugal pump?

The shut-off head of a pump indicates the amount of pressure required to reach a condition where the flow rate in a vertical pipe connected to the operating pump becomes zero.

What is cavitation of centrifugal pump?

Cavitation occurs in centrifugal pumps when the Nett Positive Suction Head Available (NPSHa) is lower than the Nett Positive Suction Head Required (NPSHr) causing the formation and accumulation of bubbles around the impeller eye that then collapse resulting in a series of mini implosions and significant damage to both ...

What is pump principle?

The basic principle of operation of centrifugal pumps is – force vortex flow. This means, when a body of liquid is acted upon by an external torque, it increases the pressure head of the rotating liquid. This increase in pressure is directly proportional to the velocity of the liquid.

What is pump warm up line?

The trick is to warm up the pump prior to opening the suction valve. An added line back from the suction side of the pump to somewhere like the flash-zone can provide a means to warm up the pump.

Why positive displacement pump is called positive?

What is a positive displacement pump? A positive displacement pump provides a constant flow at fixed speed, regardless of changes in pressure. The pump makes the fluid move by trapping a fixed amount and forcing the volume into the discharge pipe.

Why check valve is required at pump discharge?

A check valve in the discharge of any pump is to keep flow reversing through it either by gravity or when another parallel pump on the same system is operating. Specific to a sump pump, it is to keep the water being held in a discharge line in place so that the water does not flow back into the sump.

What causes a centrifugal pump to overheat?

The essential problem is simple: If a suction or discharge valve is closed or blocked, the pump runs without any fluid. Under these conditions, as much as 40% of motor power is transferred to the fluid in the pump as heat. The pump temperature rises very quickly as the fluid turns to steam that can't escape.

Why pump suction is greater than discharge?

Suction pressure diameter is usually 1 size higher than discharge, because we need minimum resistance in the suction side to reduce the losses and increase the NPSHa.

What is suction pressure of centrifugal pump?

When a centrifugal pump is taking suction from a tank or other reservoir, the pressure at the suction of the pump is the sum of the absolute pressure at the surface of the liquid in the tank plus the pressure due to the elevation difference between the surface of liquid in the tank and the pump suction less the head ...

What is priming and cavitation?

Pump priming. Pump Cavitation. Basic. It is the process of filling liquid into the suction side, pump casing, up to the delivery valve. It is the effects of the bubble formation at the suction side that carries further to the pump inside and burst, causes damage to the impeller as well as the pump.

What is valve shut off pressure?

The most common rubber tube used is natural rubber, which requires 2.5 BAR of pressure to close the valve. Other rubber membranes used such as silicon or viton can require 3 BAR of pressure to close the shut off valve.

What is shut off point?

What is Shutoff? Shutoff head is the head a pump will develop when operating against a closed discharge valve. The point where the left side a pump performance curve intersects with the Y-axis is the shutoff condition. The head corresponding to that point will be produced when the pump is producing no flow.

What is maximum discharge pressure?

The maximum suction pressure plus the maximum differential pressure that the pump is able to develop when operating with the furnished impeller at the specified speed, specific gravity, and pumping temperature.

What is the difference between centrifugal pumps and positive displacement?

Positive displacement pumps capture confined amounts of liquid and transfers it from the suction to the discharge port. In short, with centrifugal pumps pressure is created and flow results, whereas with positive displacement pumps flow is created and pressure results. Performance.