A General Guide to Pump Hydraulic Calculations

1. Introduction

This guide establishes a common design basis for performing pump hydraulic calculations during basic, front-end, and detail engineering phases. These guidelines ensure consistency and accuracy in determining pump head and suction requirements.

2. Core Principles & Inputs

Before beginning calculations, it's essential to understand the pump's function. A hydraulic sketch is highly recommended. This sketch should clearly show:

• Source and destination equipment and operating pressures.
• All relevant elevations, including equipment and liquid levels (e.g., LLLL, NLL, HLL).
• The complete piping route with all fittings, valves, and instruments.
• Calculation "nodes" at any point where the line diameter changes, flow rate changes, or a pressure estimation is required.

3. Calculation Guidelines

1. Software: Use validated engineering spreadsheets or appropriate hydraulic software for calculations.
2. Flow Rate: Use the flow rates specified in the pump's process datasheet. Do not add extra margins if the datasheet already represents the required (rated) flow. If a pump has multiple duties (e.g., different discharge routes), each case must be evaluated to find the governing scenario.
3. Slurry: Slurries exhibiting Newtonian characteristics can be treated as incompressible fluids. Use the specified slurry density and viscosity for calculations.
4. Pipe Roughness: Unless otherwise specified, use standard pipe roughness values:
   • Carbon Steel (CS): 0.047 mm
   • Stainless Steel (SS) & Alloy Materials: 0.025 mm
5. Piping Length & Fittings:
   • Use data from piping isometrics if available.
   • If not, estimate lengths and fittings from layout drawings.
   • A design margin must be added to the estimated piping length and fitting counts to account for uncertainty. The margin depends on the quality of the input data.
Typical Margins on Piping Length & Fittings
• 10% Margin: If input is from detailed piping isometrics.
• 20% Margin: If input is a line list with straight lengths and elbow counts (no isometrics).
• 30% Margin: If lengths and fittings are estimated by the process engineer from layout drawings.
7. Equipment Pressure Drops: Use the allowable pressure drops from the respective equipment datasheets (e.g., exchangers, filters). If the flow rate for the hydraulic case differs from the datasheet, prorate the pressure drop accordingly.
8. Flow Element Pressure Drops: Use pressure drops from instrument datasheets. If data is unavailable, use the typical values in Table 1.
9. Strainers: Assume a pressure loss of 0.05 bar for permanent inline strainers if vendor data is not available.
10. Fitting Losses:
    • Only include tee joints (for straight-run flow) if the branch size is more than half the main line size.
    • Treat welded tappings as a tee if the tapping size is more than half the parent line size.
    • Small-bore taps for instrumentation (PI, TI) can be neglected.
11. Fluid Properties: Variations in viscosity and density should be considered if the fluid passes through a heat exchanger or is mixed with another stream. Otherwise, variations along the pipe are typically insignificant.
Table 1: Typical Pressure Drop (ΔP) Across Flow Elements
Flow Element Type	Typical Pressure Drop (bar)
Flow Orifice	0.3
Vortex Flow Meter	0.3
Turbine Meter	0.5
Coriolis Mass Flow Meter	0.5
Variable Area Meter (Rotameter)	0.2
Electromagnetic (EMF) Meter	0.05
Thermal Mass Flow Meter	0.05
Annubar / Pitot Tube	0.05
4. Suction Side Calculations (NPSH)
1. Reference Level: Calculate and quote the Net Positive Suction Head Available (NPSHa) at a clear reference level, typically the pump suction nozzle centerline or the top of the pump foundation. This reference must be stated on the datasheet.
2. Liquid Level: Always use the "lowest specified liquid level" (e.g., LLLL or a minimum operating level) in the suction vessel for both NPSHa and differential pressure calculations.
3. Pump Elevation: If the pump centerline elevation is unknown, it can be estimated using Table 2.
NPSH Margin Requirement
A safety margin must be applied to the NPSH calculation.
• Standard Service: NPSHa ≥ NPSHr + 1.0 mlc (meter liquid column)
• Critical Service: For pumps with low available NPSH (e.g., NPSHa < 5 mlc), a larger margin is required. Use the greater of:
  • NPSHa ≥ 1.2 × NPSHr
  • NPSHa ≥ NPSHr + 1.0 mlc
Positive Displacement (PD) Pumps
For reciprocating Positive Displacement (PD) pumps, the calculated NPSHa does not account for acceleration head losses, which are vendor- and model-specific. This must be clearly noted on the pump datasheet so the vendor can account for it.
Table 2: Estimated Pump Impeller Elevation (Above Grade)
Flow Rate (m³/hr)	Estimated Impeller Elevation (m)
Up to 45	0.75
45 to 230	0.90
230 to 2300	1.05
2300 to 4600	1.35
5. Discharge Side Calculations (Head)
1. Discharge Margins: Add a design margin of 10% on the total frictional pressure drop (ΔP) of the discharge line and 1.0 mlc for static head.
2. Control Valve ΔP: Ensure the pressure drop specified for the control valve at the corresponding flow rate is included.
3. Reconciling Head:
   • If the final calculated head is *lower* than the value in the base specification, do not reduce the head. The extra head is considered a calculation margin.
   • If the final calculated head is *higher* than the value in the base specification, the specification must be revised to the new, higher value.
4. Rounding: Round the final calculated pump head *up* to the nearest 0.5 mlc. (e.g., 23.15 mlc becomes 23.5 mlc).
5. Submersible Pumps: For submersible pumps where the shaft height is unknown, quote the differential head at the pump's discharge flange. Add a note for the vendor to include the static head correction for the shaft height.
6. Parallel Pumps: For pumps operating in parallel without an auto-throttling control valve (e.g., cooling water pumps), the system resistance curve must be provided to the vendor. This ensures the "end of curve" for one pump is beyond the system resistance when all other pumps are running, preventing a trip.
7. Documentation: All assumptions made during the calculation must be clearly documented.