Pump controls and valves

The vice president of the company Mr Big boss just came to his plant for site visit and watch the plant performances and production personally.  The plant manager asked Bud (the instrument engineer) to escort Mr Big boss around the factory with him. The Big boss saw one of the pump operated and asked the plant manager what is it

Big boss: What is this thing called? This stuff is make a lot of noise and vibration.

Plant manager: This is pump boss. Pumps are used in the production facilities to move liquid from a low pressure/elevation to one of higher pressure or elevation.

Big boss: How this thing works?

Plant manager: Well, pumps are classified as either “kinetic” or “positive displacement”. In kinetic pump the kinetic (velocity) energy of the fluids first increased by rotating impeller and then converted to potential or pressure energy until it matches that in the discharge pipe (almost all of kinetic pumps are centrifugal pumps). 

Centrifugal Pumps

In a positive displacement pump (most displacement pumps are reciprocating pumps) the liquid volume is decrease and compressed mechanically by linear motion of a piston in a cylinder, causing a direct rise in potential energy until it is equal to the pressure in the discharge system.

reciprocation double piston acting pump

System pump interaction 

Big boss: Wow, So how, you’re going to reach pressure or flow liquid desired by these mechanical things.

Plant Manager: Well, We have Bud, our instrument engineer here to explain it to you. 

Bud: Ok, Basically the pump control systems range in complexity from single hand-operated valves to highly advance automatic flow control to control the flow rate, pressure, liquid level and protection for the pump and system against damage from the pumped liquid. For centrifugal pumps, either change of speed or change of valve setting can control the desired variable. For positive displacement pumps, control is by change in speed, change in setting of bypass valve, or change in displacement.

All control system have

1. Sensing or measuring element.

2. Controller

3. A final control element (a valve)

Big boss: Well, now I'm starting to get curious. Is there any rules thumb for this.

Bud: Well, if the pump operates at constant speed, there are four common control methid of obtaining the desired output such as flow rate, head, or liquid.
1. On-off: The simplest closed loop systems operate on-off between fixed limit, the simplest mechanism for on-off control is the push button switch.Centrifugal and positive displacement pumps can be controlled by this method.

2. Throttling by valve: very common and can provide refined control under difficult condition were rapid response and outstanding dynamic stabilty are sought (in boiler feedwater control for example).It can only be used in Centrifugal pumps.This pump performance curve will explain you about the control system

 System curve.

Intersection of a pump curve and system curve

(Head is a measure of resistance to flow. If a pump has a maximum output of 20 head feet, it means it can pump water 20' straight in the air, and capacity represent the flow rate)

The pump can deliver any flow rate along the curve. Under zero flow condition, there is a gravity head of liquid and pressure in container, such as a boiler pump. When liquid flows, piping friction is added. piping friction cause the system curve to turn upward. If a downstream control valve, previously wide open, is throttled, a new and more rapidly system curve is established. The intersection of a pump curve and system curve indicates condition at the pump discharge.

Big boss: Wonderful, I think you deserve a good rating  in your performance review this year.

Bud. Thanks boss.

After the vice president leave, Weary but happy, Bud head back to his office

References:

1. Maurice Steward , Surface production operation

2. W O'Keefe, Pump controls and valves