In one project,we used one fire water pump with capacity about 350m3/h, 120mlc, the power rate of the motor is 220Kw,the discharge pipe diameter is DN300. The check valve between the pump is about 2meter. The pump is under the water line, so the piping between the check valve is full of water. The pressure in the downstream piping of the check valve is always remain 12Bar. We need the fire water pump start with the discharege valve of the pump fully opened and all the fire hydrant and the spray are closed, but in fact the pump can not be started. Beacaue the motor is a little small and the starting time is too long.We have tried many times,and every time the swith board was disconnected. so very time, we have to close the discharge valve to start the pump, and then open it.
In final, we installed a orifice plate in the outlet of the pump, and this time, the pump can be started with the discharge valve fully opened.
Question 1:
     Can anybody tell me what the difference between the pump start without the orifice plate and the pump start with the orifice plate?
The piping system is full of water,all the hydrant is close. when the pump start, it is also like the close discharge valve to start, but why it can not be started?

Question 2:
for the fire water pump start with the discharge valve fully open, how much power rate should be fit for the pump?
Is there any empirical equation to calculate?

The motor capacity seems to be sufficient even if your pump efficiency is about 60%. Actually you should take the help of a good electrical engineer in this regard.

As far as the fire hydrant system is concerned, I have a few suggestions.

1. As far as my experienced is concerned, your system is ok if your fire hydrant pump starts once in every hour, otherwise take a look on system leakage.

2. Generally jockey pumps assist main hydrant pumps to maintain required pressure when the system is not open. This helps you in reducing power consumption.

3. The flow capacity of the hydrant pumps is decided with respect to the number of hydrant points that can be opened at a time. This depends upon your local codes. That is why you are not supposed to play with the discharge of the pumps. I highly recommend not to place any flow control devices in the pump discharge.

BTW, what is the line pressure after check valve just before your pump is being started?

The pressure in the piping after the check valve is always maintains about 12 Bar. No leakage in the piping system. The maxium power cumsuption of the motor is 200Kw at 150% capacity. motor power rating is 220Kw. And the pump is starting direct on line.
Power supply for the motor is 3 Ph./ 380 volts/50Hz. And the rated current for the motor is 410Amps, Starting currents is 2580Amps.the fire pump is using DOL start. We using Auto-transformer , which has tappings for 270V, 250V and 230V. Using the 230V tapping, the starting current will be approximately 1090A.

for this project, it is only a example.
My question is aim at all the motors of the fire water pumps.
For the fire water pumps, if we need to start the pump directly on line with the valve fully open, how should we choose the moter.

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