Practical Process Engineering

It is a monotonically increasing function in first quarter starting from a positive Z. Based on the curve shape, its slope (rate of suction velocity change in terms of suction level change) is a little bit higher at low Z and then it becomes almost constant for ever. The fluid velocity at pump suction rises always by any level increase if there is no control on pump flow rate. Although it increases head loss at pump suction, it should be carefully case studied to find out how significant it is compare with the positive effect of suction level . If head loss exceeds the amount of level increase, pump suction head drops and discharge pressure drops as well. But, if level increase exceeds the suction head loss, pump suction head grows and, then, there would be different conditions. Pump discharge head is the summation of suction head and pump head. Please note that Pump head falls if flow rate increases. Therefore, the pump curve should be reviewed to figure out how much is the pump head loss. If the suction head increase exceeds the pump head loss, so the discharge pressure increases and if the suction head increase is being equalized by the pump head loss, discharge pressure would be constant and if the pump head loss is significant, the discharge pressure decreases. All in all, if there is no control over the pump flow rate, different senarios may show up as a result of suction level change. Pump performance curve should be reviewed and the amount of head loss in suction pipe should be calculated and compared with the positive effect of level increase.

The velocity changes directly by the suction level (Z). What is your idea about the sign of the left term under the radical? Is it a positive term or negative? It MUST be a negative number, because the function would start from the left side of y-axis if it is a positive number. Accordingly, vessel pressure is always less than pump suction static pressure numerically. How would be the shape of the function of velocity in terms of the suction level? See the below figure.

2-The effect of suction level on the pump discharge pressure: A) If flow rate is controlled at a fixed set point, velocity and, thus, developed pressure drop at pump suction are constant. So, the suction head increases and results in higher discharge pressure at fixed pump head. B) What if there is no control on pump flow rate? Under this condition, higher suction level leads to higher suction velocity and more flow rate through the pump. Let’s take a closer look at the Bernouli equation and derive a relation for velocity in terms of suction level.

***The effect of pump flow and suction level at discharge pressure for regular pump (not lifting pump) 1-The effect of flow on discharge pressure at fixed suction level: Fluid velocity is affected by flow directly. If flow increases, velocity rises and suction head loss increases as well. Thus, suction head decreases. Pump head (differential head) is difference between discharge head and suction head. So, the discharge head is the summation of suction head and pump head. Normal pump performance is such that the pump head drops as flow increases. Therefore:

1-Pump suction is the reference level 2-velocity at liquid surface is zero. 3-Let’s denote Z2 to Z. 4- The last term is the total head loss (minor + major)

Bernouli equation is applied between points 1 and 2:

Where: 1- First term is the stagnation pressure head at pump suction. P is the static pressure or thermodynamic pressure. It is divided by specific weight to be expressed in head. Dynamic pressure is developed by fluid velocity and a static fluid doesn’t have it. 2- Second term is vapor pressure head.