Pump Design Using Aspen HYSYS: NPSH, Pump Curves and Sizing Explained
Quick answer: Pump design means choosing a pump that delivers the flow and head your process needs, with NPSH available above NPSH required and the operating point close to the best efficiency point (BEP). In Aspen HYSYS you set up the fluid and the pump, read the power and fluid properties, and check NPSH available with a spreadsheet. The pump vendor then selects the real pump from your duty and its own curve.
In this article
- 1. What are the main type of pumps?
- 2. What are NPSH and cavitation?
- 3. How do pump curves and system curves set the operating point?
- 4. How do you size a pump?
- 5. How do you simulate a pump in Aspen HYSYS?
- 6 Worked example
- 7. Join the CHEMKLUB pump design webinar
- 8. Frequently asked questions
(introduction)
Picking a pump looks simple: match the flow and move on. In practice, a pump that is too big wastes energy, one that is too small misses the duty, and one with too little suction pressure damages itself through cavitation. This article is for chemical and process engineering students and working process engineers who want a clear method. You will learn the main pump types, how NPSH and cavitation work, how pump curves and system curves set the operating point, how to size a pump and its motor, and how to simulate a pump in Aspen HYSYS. A worked example with real numbers is included. CHEMKLUB India covers the same topics in a live webinar on 31 October 2026.
What are the main types of pumps?
Pumps fall into two groups. Centrifugal pumps use a spinning impeller to add speed and pressure to the liquid. They suit large flows of thin liquids such as water, light hydrocarbons and many chemicals, and most plant pumps are of this type.
Positive displacement pumps trap a fixed volume of liquid and push it forward. Gear, screw, lobe, piston and diaphragm pumps belong here. They suit thick liquids, high pressure at low flow, and accurate dosing.
A simple rule: for large flow and a thin liquid, choose a centrifugal pump. For a thick liquid, or a flow that must stay steady as pressure changes, choose a positive displacement pump. For a small, exact flow such as chemical dosing, choose a diaphragm or metering pump.
What are NPSH and cavitation?
NPSH (net positive suction head) is the pressure margin the liquid has at the pump inlet before it starts to boil. It is written in metres of liquid. NPSH available (NPSHA) comes from your system: vessel pressure, liquid level, suction pipe losses and temperature. NPSH required (NPSHR) comes from the pump curve and rises as flow rises. NPSHA must stay above NPSHR, with a safety margin.
NPSHA = (suction pressure − vapour pressure) ÷ (density × g) + static head − suction line losses (use absolute pressure)
Cavitation happens when the inlet pressure falls below the vapour pressure of the liquid. Vapour bubbles form and then collapse inside the pump, which causes noise, vibration, lower flow and pitting of the impeller.
Heat is the usual trap. With the same layout (open vessel, liquid 2 m above the pump, 0.5 m suction loss), water at 40 °C gives an NPSHA of about 11.2 m, but water at 90 °C gives only about 4.8 m.
How do pump curves and system curves set the operating point?
The pump curve comes from the manufacturer. It shows head against flow, with efficiency, power and NPSHR on the same chart. The system curve shows the head your piping needs at each flow: static head plus friction losses, and friction grows roughly with the square of flow.
The pump works where the two curves cross, called the operating point, not at the catalogue duty point. Keep it close to the best efficiency point (BEP). Many specifications ask for 70% to 120% of BEP flow (the API 610 preferred operating region), so check your own.
Far left of BEP, pumps heat up, recirculate inside and vibrate. Far right, NPSHR rises and the motor can overload. A control valve on the discharge raises the system curve, so oversizing a pump and then throttling it wastes energy.
How do you size a pump?
Sizing follows five steps:
- Fix the duty. Note the normal and maximum flow, liquid, temperature, density, viscosity and vapour pressure.
- Find the head the system needs. Add the height difference, the pressure difference between destination and source, the friction losses in pipes, fittings and equipment, and the control valve drop.
- Check NPSHA at the worst case. Use the lowest liquid level, the highest temperature and a partly dirty suction strainer.
- Pick the pump. The pump curve must pass above the duty point, and the operating point should sit close to BEP.
- Calculate power. Hydraulic power (kW) = flow (m³/h) × differential pressure (bar) ÷ 36. Shaft power = hydraulic power ÷ pump efficiency. Then choose a motor with the margin your specification asks for.
Common mistakes: choosing on flow alone and ignoring head, checking NPSHA at normal instead of worst-case conditions, treating NPSHR as one fixed number, forgetting strainer and fitting losses, and ignoring viscosity. Thick liquids give lower head, lower flow and lower efficiency than the water curve shows.
How do you simulate a pump in Aspen HYSYS?
Aspen HYSYS is a process simulator from AspenTech. It does not pick a pump model from a vendor catalogue. It gives you the process side: fluid properties, stream conditions and pump power. A simple workflow:
- Set up the case. Add the components and choose a fluid package that suits your liquid. It controls density and vapour pressure, so it directly affects NPSHA.
- Create the feed stream with temperature, pressure, flow rate and composition.
- Add a Pump from the Object Palette. Connect the inlet stream, the outlet stream and an energy stream.
- Specify the pump. Enter the outlet pressure (or pressure rise) and the efficiency. HYSYS calculates the power, and the energy stream carries the shaft power.
- Get the vapour pressure. Copy the suction stream, keep the same temperature and set the vapour fraction to 0. HYSYS then reports the bubble point pressure.
- Calculate NPSHA. Put the formula in the HYSYS spreadsheet so it updates with every change. If you have the manufacturer’s curve, enter it in the rating section of the pump to see behaviour away from the design point.
- Run what-if cases. Use the case study tool to change flow, temperature and suction pressure, and watch power and NPSHA move.
Worked example
A pump moves 50 m³/h of water at 40 °C against a differential head of 60 m. What power and motor does it need, and is NPSH safe?
How the numbers come out: differential pressure = 992 × 9.81 × 60 ≈ 584,000 Pa = 5.84 bar. Hydraulic power = 50 × 5.84 ÷ 36 ≈ 8.1 kW. Shaft power = 8.1 ÷ 0.70 ≈ 11.6 kW, so an 11 kW motor is too small and the next standard size is 15 kW, before any extra margin your specification asks for. NPSHA = (101.3 − 7.4) kPa ÷ 9.73 kPa/m + 2 m − 0.5 m ≈ 11.2 m.
Now check the hot case. At 90 °C the same layout gives an NPSHA of about 4.8 m, a margin of only 0.8 m over the 4.0 m required. That is too tight, so you would raise the liquid level or cool the liquid.
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Join the CHEMKLUB pump design webinar
CHEMKLUB India is running a live webinar, Pump Design using Aspen HYSYS, on 31 October 2026 from 8:00 PM to 10:00 PM IST. It pairs with our Aspen HYSYS training and is built for students, fresh graduates and working process engineers. The session covers:
- Fundamentals of pumps: design, classification and applications
- NPSH and cavitation
- Pump curves and the system curve
- Pump selection and sizing
- Pump simulation in Aspen HYSYS
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