IV. PUMP APPLICATIONS
E. Sizing a WGI Pump for High Inlet Pressure Conditions
1. For double acting piston pumps, no derating is required.
2. For a single acting plunger pump, where the rod load due to inlet pressure is less than 20% of full plunger/rod load, no derating required.
3. For a single acting plunger pump (with standard bronze wrist pin bushings), where the rod load due to inlet pressure is greater than 20% of full plunger/rod load, derating is as follows:
a. For triplex pumps, add two-thirds the inlet/suction pressure to the discharge pressure in selecting the pump maximum plunger pressure.
b. For quintuplex pump, add two-thirds the inlet/suction pressure to the discharge pressure in selecting the pump maximum plunger pressure.
4. For a single acting plunger pump (with optional wrist pin roller/needle bearings), where the rod load due to inlet pressure is greater than 20% of full plunger/rod load, no derating is required if both of the following requirements are met;
a.) inlet/suction pressure is less than 50% of the maximum plunger pressure rating
b.) discharge pressure is less than the maximum plunger pressure rating.
V. SUPPLY SYSTEM CONSIDERATIONS
A. Pressure
Liquid pressure is defined as the normal component of force per unit area. In common practice and general function, pressures are frequently measured in pounds force per square inch (lbf/in2). Gauge pressure (psig) is the difference between absolute pressure (psia) and the atmospheric pressure (Pa). Appendix A, Table 7 shows the relationship between atmospheric pressure and elevation. Vapor pressure is the absolute pressure exerted by the liquid and its vapor to maintain an equilibrium condition at a given temperature of the liquid.
Example 7:
Find vapor pressures of water at 76°F and 212°F at sea level..
From Appendix E - Table of Vapor Pressure of Water:
The vapor pressure of 76°F water is 0.4443 psia at sea level.
The vapor pressure of 212°F water is 14.696 psia at sea level.
B. Head
The English unit for measuring head is feet. The equation, expressing pressure (psi) in units of feet, is:
Head = psi x 2.31
S.G.
Where,
S.G. = Specific Gravity @ pumping temperature.
Example 8:
Find the head in units of feet (ft.) of crude oil, with a S.G. = 0.8 @ pumping temperature, at 20 psi pressure.
Head = 20 x 2.31 = 57.75 ft.
0.8
Example 9:
Find the head in units of feet (ft.) of mercury with a S.G. = 13.6 @ pumping temperature), at 20 psi pressure.
Head = 20 x 2.31 = 3.39 ft.
13.6
C. Viscosity
Basic metric viscosity units are the poise (absolute/dynamic viscosity) and the stokes (kinematic viscosity). More customary expression of these units are centipoise and centistokes respectively, each equal to 1/100th the of basic metric viscosity unit. The relationship between the English units for medium viscosity liquids, SSU (Saybolt Universal Seconds), and metric absolute viscosity is:
n (absolute viscosity, centistokes) = 0.22 (SSU) - 180
(SSU)
Introducing the mass density of the liquid (r) allows the expression of the relationship between absolute viscosity to Kinematic viscosity as follows:
m (Kinematic viscosity, centipoise) = rn
Example 10:
Find viscosity in centistokes (n) and centipoise (m) of a liquid with S.G. = 0.8 with a viscosity of 500 SSU.
n = 0.22 (500) - 180 = 109.64 centistokes
500
Then,
m = 0.8 x 109.64 = 87.71 centipoise
The basic pump speed, and its relationship with various ranges of liquid viscosity, are discussed in further detail in Appendix A.
D. Frictional head losses
Pipe, valves, fittings, hoses, and meters installed in the liquid supply piping system generate resistance to the liquid flow. The friction head is the hydraulic pressure required to overcome frictional resistance of a piping system. The Table in Appendix C shows an equivalent length in feet, of 100 percent opening valves and fittings. Pressure drop in liquid lines versus liquid flow rates is shown in Appendix D.
E. Reynolds Number
The Reynolds Number (Re) is used in closed conduit/pipe flow, deals with the viscous force in a liquid, and is defined by the following equation:
Re = r1 df n1
m1
Where,
Re = Reynolds Number
r1 = liquid density at flowing temperature, lbm/ft3
df = pipe inside diameter, feet
n1 = liquid flow velocity, ft/sec
m1 = liquid viscosity
(centipoise divided by 1488 or centistokes multiplied by S.G. then divided by divided by 1488)
Customarily; turbulent flow occurs when the Re is greater than 3000, laminar flow occurs when the Re is less than 2000. The transition period is when the Re is between 2000 and 3000.
Example 11:
A 14" schedule 30 piping system is designed to deliver 18,970 BPD (553.3 GPM) of crude oil with a Kinematic viscosity of 50 centistokes and S.G. = 0.8 @ 100°F.
Find the Reynolds Number (Re).
Re = r1
df n1
= 49.92 x 1.104 x 1.28
m1 0.02688
Re = 2624.2
Where,
r1 = 62.4 x 0.8 = 49.92 lbm/ft3
df = 13.25 = 1.104 feet
12
n1 = 553.3 = 1.28 ft/sec
2.45 (13.25)2
m1 = 50 x 0.8 = 0.02688
1488
