Introduction
Pumps are common pieces of process equipment. How difficult are they to specify? Our process simulator tells us most of what we need to know (not including specialized metallurgy to combat corrosion):
- What is the required flowrate?
- What is the required pump head?
- What are the fluid properties?
Often, this is the only information that an engineer provides to a pump vendor. Equally as often, the engineer is surprised when the vendor says the pump application is not feasible.
This article looks at how we can use process requirements (flowrate, head) to evaluate pump design characteristics.
Specific Speed: Head and Stages
First, we will look at pump head and the required number of impeller stages.
The Specific Speed is a pseudo-dimensionless number for the pump head that is generated at the Best Efficiency Point. Refer to Reference (5) for a good description of Specific speed.
where RPM is Revolutions per Minute, Q is flowrate in USGPM, and H is the pump head in feet. The specific speed represents the shape of the impeller.
Typical centrifugal pump efficiency map, see Ref (1).
We know the process flow rate and head. We can choose the pump speed (typically 1800 or 3600 RPM) and then calculate the Specific Speed. Then we can use the efficiency map to find the efficiency. If the flowrate and Specific speed are not on the map then our pump may not exist. We can change the number of pump stages and find the pump that will exist.
Can we use basic process design data to tell us anything about the NPSH requirements, or the minimum flow requirements? Yes.
Suction Specific Speed: NPSH
Suction Specific Speed was defined by Karassik in the 1930’s. This is a dimensionless form of the NPSHR at the Best Efficiency Point.
Pumps with low NSS have a smaller suction eye, and this is associated with higher NPSHR. A larger impeller eye results in lower NPSHR and thus higher NSS.
Centrifugal pump impeller eye diameter and Suction Specific Speed, see Ref (2).
Suction Specific Speed is a design parameter for a centrifugal pump. Most centrifugal pumps in petrochemical plants are designed to have NSS between 5000 and 15,000 (typically). Since process engineers are often concerned about NPSH limits for pumps, why would we select a pump with a low NSS (with a high NPSHR) where we could select a pump with a high NSS (and provide a low NPSHR)?
It turns out that NSS also tells us something about the preferred operating range (turndown) for the pump. Here is one map:
Sample map for centrifugal pump preferred operating range, from Process Industry Practices Ref (3).
This is a better map because it spells out the Minimum Continuous Stable Flow as a function of the pump Specific Speed:
Better map for centrifugal pump minimum flow, from Joe Evans Ref (4).
Pumps with a lower NSS have a lower Minimum Continuous Stable Flow than pumps with a higher NSS. If we operate the pump outside the preferred range then we can expect operating problems.
The Hydraulic Institute recommends NSS < 8500 for a reliable pump. I have seen NSS = 7500 as a customary design point for reliable operation. Careful design and operation is needed with NSS over 11,000, and I have seen boiler feed pumps with NSS = 15,000.
Example
Suppose we have a pump with Specific Speed NS = 2500 and we select a pump with NSS = 7500. Our graph tells us the pump will have a minimum continuous rated flow of 40%. This pump has fairly wide range of flowrate for reliable operation.
But what if this requires an unacceptable amount of NPSHR? This is often the case for a Deaerator that supplies saturated water to a boiler feed water pump. Lets pick a pump with NSS = 15,000: We can expect the MCSF to be 60% of the BEP flow rate. This operating range is considerably narrower than a pump with lower NSS.
Recommendations
1. Always start with a low NSS pump, say NSS = 7500. Calculate the required NPSH and the MCSF.
2. If the required NPSH is excessive, then assume a higher NSS, such as 13,000. Calculate the required NPSH and MCSF.
3. Be aware that high NSS pumps are less tolerant to upsets and we need to pay close attention to piping layout.
References
1. Terry Henshaw, Centrifugal Pump Specific Speed, Jan 18, 2012, https://www.pumpsandsystems.com/centrifugal-pump-specific-speed
2. Ron Astall, https://www.pumpindustry.com.au/understanding-pump-curves-6-minimum-flow/
3. PIP REEP007, Guidelines for Minimum and Maximum Flow Rates for Centrifugal Pumps
4. Joe Evans, Suction Specific Speed & Wastewater Pumps, Oct 21, 2013, https://www.pumpsandsystems.com/suction-specific-speed-wastewater-pumps
6. Terry Henshaw, Suction Specific Speed: Part 1, Dec 17, 2011, https://www.pumpsandsystems.com/suction-specific-speed-part-one
