Calculation suction specific speed

This operating window is quantified or rated by the term Suction Specific Speed, Nss . The Nss is calculated with three parameters, the speed, the flow rate, and the NPSHr. These numbers come from the pump s performance curve, discussed in Chapter 7. The formula is the following ... 

Annex A specifies calculations for specific speed and suction specific speed. 

The pump suction specific speed shall be calculated in accordance with Annex A and, if specified, limited as stated on the data sheet. 

Answer by Author A criterion for NPSH computation reliability is its repeatability when using a wealth of data. A three-thermocouple redundancy per measurement and the multiplicity of test runs pinpointed the temperatures selected. Then, by using equivalent vapor pressures, and by judiciously establishing pump inlet head losses, no erroneously-negative values of NPSH were realized. Since related suction specific speed values were neither imaginary nor infinite, and, because no hypothetical approaches were necessary to calculate their real values, the NPSH values were deemed reliable. 

In order to make the correlation described in this paper, it is necessary to perform the calculation at a point where the curves become vertical, in other words, the point where the pump is fully cavitating. For this unit the difference in NPSH is 0,6 foot, Calculations using only the water data show that the unit should perform satisfacto rily with liquid oxygen requiring a NPSH which is 0,6 foot lower than that needed for water. It is noteworthy that the corresponding difference in Suction Specific Speed values is a factor of 2,... 

Based on above suction specific speed guidelines and Bq. (10), maximum allowable pump speed can be calculated. In general, a pump with higher speed will be smaller (less expensive). On the other hand, a lower speed pump is more reliable. Therefore, we need to consult pump vendor to select a high speed, but also reliable pump. 

Suction specific speed deals with pump inlet condition and it is calculated by following equation. 

For double suction pumps, using the HI convention, is taken as the total pump flow, although some pubHcations use half-flow, ie, flow per impeller eye. For multistage pumps, the developed head must be taken per stage for the NS calculation. By definition (eq. 9), high head, low flow pumps have low specific speed low head, high flow pumps, such as turbine and propeller pumps, have high specific speed. 

It has played a dual role, one in Equation 2.18 on specific heat ratio and the other as an isentropic exponent in Equation 2.53. In the previous calculation of the speed of sound. Equation 2.32, the k assumes the singular specific heat ratio value, such as at compressor suction conditions. When a non-perfect gas is being compressed from point 1 to point 2, as in the head Equation 2.66, k at 2 will not necessarily be the same as k at 1. Fortunately, in many practical conditions, the k doesn t change very... 

I like to calculate the relative compression efficiency because I do not have to know the flow of process gas. I do not have to know the driver horsepower output, the steam to the turbine, the fuel gas to the gas turbine, or the speed of the compressor. I do not have to know Z (the gas compressibility factor) or K (the ratio of the specific heats). The things I do have to know—the suction and discharge temperature and pressure—I can check with my own hands and my own tools. 

After consulting Ref. P3 (p.582 Figure 14.2), it is confirmed that the high suction pressure available in this application ensures that cavitation will not occur in the pump. For the calculated specific pump speed, there is a safety factor of over 500%. Cavitation should not be a problem provided that the column is only partially repressurised before turning on the pump (a usual start-up procedure on the absorption column). 

N is pump speed, in rpm Q equals total pumping rate, in gpm. (for double suction pump, in USA, total flow is used for specific spenl calculation, while in Europe, half flow is used.) [1] and H is head per stage, in ft. These values should always take at maximum pump impeller diametm and at Best Bftictency Point (BEP). 

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