What is Fire jockey pump | How to calculate capacity of Fire jockey pump?

Fire jockey pump maintains the designed pressure (in fixed range) in the fire water network to compensate the minor leakages and to avoid non-emergency starting of the main fire pump.

Fire water jockey pump requirement and selection

What is Fire jockey pump | How to calculate capacity of Fire jockey pump?

The fire water system is designed for a minimum residual pressure of 7.0 kg/cm2 g at the hydraulically remotest point of application at the designed flow rate at that point. To meet this requirement, fire water network is designed to maintain minimum static pressure somewhat higher than 7 Kg/cm2 in piping network, it depends on total fire water demand and length of piping network. To keep the fire water network always pressurized at designed pressure, jockey pumps are provided.

 

Capacity of Fire Jockey Pump

There should be two Fire jockey pumps (one main and one standby) at Fire water pump house other than the main fire pumps. These two jockey pumps must be in auto mode.

The capacity of the jockey pumps must be sufficient to compensate the desired fire water system pressure in the event of any minor leakages from valves, piping or operation of single hydrant etc. to avoid non-emergency starting of the main fire pump.

The capacity of jockey pumps should not be less than 5% and should not be more than 10% of the design fire water rate. As these pumps maintains the header pressure in non-emergency situation and therefore it’s head shall be higher than the main fire water pumps. The suction pressure may be minimum 0.3 kg/cm2 and Maximum 1.06 kg/cm2.

Pump must be capable of discharging 150% of its rated capacity at a minimum of 65% of the rated head. The shut-off head should not exceed 120% of rated head, for horizontal pumps and 140% in case of vertical turbine type pumps.

For example;

Suppose the total fire water demand is calculated as 2400 m³/hr then the capacity of each jockey pump will be 120 m³/hr (5% of total Fire water demand).

And suppose the designed header pressure is calculated as 10.5 kg/cm² to be maintained in the network all time (considering the size, length and fiction loss, etc) to meet the criteria of minimum 7 kg/cm² at the hydraulically remotest point of application at the designed flow rate at that point, then the maximum design pressure of jockey pumps may be 11.8 kg/cm2 and differential head at rated capacity will be 115 MWC (NPSH is 10.5 MWC).

For above example, Jockey Pump operation parameters will be

(a) Normal rated capacity = 120 m3/hr

(b) Suction pressure = 0.3 to 1.06 kg/cm2

(c ) Discharge pressure = 11.8 kg/cm2

(d) Differentials pressure = 11.5 kg/cm2

(e ) Differential Head = 115 MWC (at discharge flange)

(f) NPSHA = 10.5 MWC

(g) Hydraulic power = 42 kw

 

Fire Jockey pumps sequence of Auto Operation

Jockey pumps should be maintained in auto mode and operation philosophy should be as per designed pressure. Both the jockey pumps should be set for auto cut-in and auto cut-off facility. Also, if the main fire water pump starts then the jockey pump must be stop automatically. Pumps must start automatically and sequentially with PT/ PLC on fire water mains. The system ensures auto start of the standby pump in case a pump in sequence failed to take start. In the event of failure of main jockey pump, standby jockey pump should start after 30 seconds as per pre-defined logic sequence.

For example:

Suppose, the designed header pressure is calculated as 10.5 kg/cm2 to be maintained in the fire water network at all time then the jockey pump operational sequence may be set as; auto start at 10 kg/cm2 and auto stop at 10.5kg /cm2.

 

Power supply to Fire Jockey pumps

Power supply through a direct feeder dedicated only to fire pumps must be ensured from the sub-station to ensure reliable power supply. Emergency power supply must be provided for jockey pumps to deal with black out event. In many industries, Electrical diesel generators are connected with jock pumps. Both the pump should be provided with manual ON/OFF switches and remote selection switch on controller.