Fire Fighting Pump Calculation

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1. DESIGN INPUT

Pump operating sketch for Fire Fighting Pump is shown below:



Figure.1 Pump operating schematic

Design input for fire fighting pump calculation is shown in table 1 below:

Table.1 Design Input for Pump Calculation
Parameter
Value
Temperature @ fire water pond
32oC
Vapor pressure
-0.95 bar.g
Viscosity
1 cP
S.G.
1
Pump capacity
227.1 m3/hr
Capacity over design
227.1 m3/hr
Max. destination pressure
7.9 bar.g
Pump efficiency (assumed)
100%
Suction line diameter
8 inch
Discharge line diameter
8 inch
Min liquid level @fire water pond
-2800 mm
Ground to base plate elevation
-1800 mm
Suction pump nozzle elevation
250 mm
Discharge pump nozzle elevation
450 mm
Maximum discharge elevation
1000 mm
Suction line length (+25% margin)
3.75 m
Discharge line length (+25% margin)
375 m



2.    CALCULATION INSTRUCTION

The following Steps are used for equipment sizing of Pump:

STEP-1 SUCTION PRESSURE CALCULATION
1.    Determine the Minimum Vessel Pressure or pit.
Minimum Vessel Pressure is a minimum pressure at suction vessel during pumping the liquid. In many cases, it’s called as suction vessel pressure.
2.    Calculate the Minimum Static Head (MSH)/ lift  by using formula as follow:

Where:
DSH          =  Differential Suction Head (m)
h1   =  Minimum elevation of vessel level (m)
h2   =  Elevation of pump suction (m)

Where:
MSH          =  Minimum Static Head (bar)
DSH          =  Differential Suction Head (m)
Sp.Gr        =  Specific Gravity
3.    Friction Loss Calculation
Friction loss is loss of pressure that caused by friction of fluid through pipes, fittings and equipments. Detail fittings in suction pump are shown in table 2 below:

Table.2 Fitting Details in Pump Suction
Fitting @suction
Quantity
Gate Valve
Swing Check Valve
Elbow 90o
Reducer
Y strainer
1
1
4
1
1

Calculate the Friction Loss by using formula as follow:
Where
ΔP =  Friction loss (bar)
fd   =  Darcy friction factor
ρ    =  Density (kg/m3)
L    =  Pipe length + Equivalent length from fittings (m)
v    =  Velocity (m/s)
ID  =  Internal Diameter (mm)

4.    Calculate Suction Pressure (SP) by using formula as follow:
Where
SP =  Suction Pressure (bar.g)
MVP          =  Minimum Vessel Pressure (bar.g)
MSH          =  Minimum Static Head (bar)
ΔP =  Friction loss in suction (bar)

STEP-2 NPSH AVAILABLE CALCULATION
NPSH (Net Positive Suction Head) is used to indicating the condition/ability of pump suction. There are two types of NPSH, NPSHA (available net positive suction head) and NPSHR (required net positive suction head). NPSHA is the differential pressure between actual pressure in the suction nozzle of the pump and vapor pressure of the liquid. Calculation Steps of NPSHA are:
1.    Determine the vapor pressure (PV in bar.g) is vapor pressure from the liquid at pump temperature, usually is shown as absolute pressure
2.    Determine liquid flow rate. The worst case scenario for liquid flow rate is when 2 pump running. Conservative assumption is using 1.5 design flow rate for pressure drop evaluation in NPSHa calculation.
3.    Calculate the Suction Pressure (SP) using 1.5 design flow rate
4.    Determine the NPSH Available
Where
NPSHA     =  Nett Positive Suction Head (m)
SP             =  Suction Pressure (bar.g)
VP             =  Vapour Pressure (bar.g)
Sp.Gr                    =  Specific Gravity

Result of NPSHA calculation and NPSHR information from vendor are useful to preventing the pump from cavitation. Cavitation is happened when the fluid pressure at suction impeller is below than saturated vapor pressure of fluid.

STEP-3 TOTAL DISCHARGE PRESSURE CALCULATION
1.    Determine of Maximum Destination Pressure (MDP)
   Maximum Destination Pressure is maximum pressure at discharge vessel during pumping the liquid. In many cases, it’s called as discharge vessel pressure.
   Calculate the Discharge Static Head/ lift (DSH)
Where:
DDH          =  Differential Discharge Head (m)
h1   =  Elevation from discharge point (m)
h2   =  Elevation of discharge pump (m)

Where:
DSH          =  Discharge Static Head (bar)
DDH          =  Differential Discharge Head (m)
Sp.Gr        =  Specific Gravity

2.    Friction Loss Calculation
Friction loss is loss of pressure that caused by friction of fluid through pipes and equipments, such as strainer, instrumentation and fittings. Detail fittings and instrument in pump discharge is shown in table 3 below:

Table.3 Fitting Details in Pump Discharge
Fitting @discharge
Quantity
Gate valve
Check valve
Tee
Elbow 90o
Enlargement
1
1
10
10
1

Calculate the Friction Loss by using formula as follow:
Where
ΔP =  Friction loss (bar)
fd   =  Darcy friction factor
ρ    =  Density (kg/m3)
L    =  Pipe length + Equivalent length from fittings (m)
v    =  Velocity (m/s)
ID  =  Internal Diameter (mm)

3.    Calculate Total Discharge Pressure (TDP) by using formula as follow :
Where
TDP           =  Total Discharge Pressure (bar.g)
MDP          =  Maximum Destination Pressure (bar.g)
DSH          =  Discharge Static Head (bar)
ΔP =  Line friction loss (bar)
ΔPother=  Additional friction loss by Orifice, HE, or control valve (bar)

STEP-4 BRAKE HORSE POWER CALCULATION
Brake Horse Power is used to indicating the requirement power of pump. Calculation steps of Brake Horse Power are:
1.    Determine the pump efficiency
Pump efficiency is capacity of pump to convert power from fuel. Usually is shown as percentage.
2.    Calculate Head
Where
DP             =  Discharge Pressure (bar.g)
SP             =  Suction Pressure (bar.g)
Sp.Gr                    =  Specific Gravity
3.    Determine Hydraulic Horse Power
Where
HHP                      =  Hydraulic Horse Power (kW)
Q               =  Flow rate (m3/hr)
Sp.Gr                    =  Specific gravity
Head                     =  differential elevation (m)
η                =  Efficiency pump (%)

STEP-5 MAXIMUM SHUT OFF PRESSURE
1.    Determine Maximum Vessel Pressure (MVP) is a maximum pressure at suction vessel during pumping the liquid. In many cases, it’s called as suction vessel pressure.
2.    Calculate Maximum Suction Pressure
Where :
MSP          = Maximum Suction Pressure (bar.g)
MVP          = Maximum Vessel Pressure (bar.g)
MLP          = Maximum Liquid Suction Pressure (bar.g)
ΔP = Line Loss (bar.g)
3.    Calculate Shut Off Pressure
Where :
SOP          = Shut Off Pressure (bar.g)
MSP          = Maximum Suction Pressure (bar.g)
DP = Discharge Pressure (bar.g)
SP = Suction Pressure (bar.g)

3.    RESULT

The result summary for fire fighting pump calculation is shown below:
-          Suction pressure                    : -0.28 bar.g
-          Discharge pressure    : 8.72 bar.g
-          NPSH available                      : 6.66 m
-          Hydraulic Horse Power          : 55.68 kW (100% pump efficiency)

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