Fluid Mechanics for Chemical Engineers Arif Hussain (Lecturer) FLUID MACHINES Fluid Mechanics for Chemical Engineers Arif Hussain (Lecturer)

Pumping (Basic Terminology) In a pumping system, the objective is to transfer a liquid from a source to a destination. This may be filling a reservoir at a higher level or circulating liquid as in a heating system. In either case a pressure is needed to make this happen. This is generally referred to as HEAD. Static Head Friction Head

Static Head It is the vertical distance that the liquid has to be lifted in order to achieve useful work.

Friction Head The friction head, which may be due to the materials of the pipe, the size of the pipe, is basically increasing as a square of the increase in flow.

Friction Head The friction head, which is basically increasing as a square of the increase in flow.

System Head/ System Curve When we put these together, and we add a static head and friction head, we end up with a system curve. System curve is where we need to look if we are going to save money and save energy with a pumping system

System Head/ System Curve Where the static head is relatively high compared to the friction head there is actually less capability of saving money but you are still going to actually save some.

Pumping Mechanism

PUMPS Pumps are used to force a liquid to flow from a point of low pressure to one of higher pressure. There are two general classifications of pumps. 1).Centrifugal Pump 2).Positive Displacement Pump

Centrifugal Pumps (Basic Pump Parts) A typical centrifugal pump has five basic parts Casing Impeller Shaft Bearings Seal or Packing

CASE Visible part of the pump Other parts are enclosed within it Usually made of cast iron, steel, plastic etc In oilfields, casing on pump operating at a pressure below 1000 kPa made of cast iron. Higher pressure operating pump generally will have a casing made of steel.

Impeller Causes liquid pressure to rise. Firmly attached to the shaft, rotates inside the case at the speed of the shaft. Most oilfield impellers made up of cast iron. Closed vane develops higher pressure but has a lower capacity Open vane develops lower pressure but has a higher capacity.

Shaft The shaft rotates inside the case at the speed of the driver. It usually made of steel. The portion of shaft exposed to the seal or packing may have a sleeve made of hard metal, such as tungston carbide, to resist corrosion or wear at that point.

Bearings Bearings serve two functions on a pump: To hold the shaft so that it does not wobble inside the pump casing. To prevent lateral movement of the shaft so that the rotating parts do not touch the pump casing.

Seal or Packing The seal or packing is used to prevent liquid under pressure inside the pump from leaking out the pump. Mechanical seal is often used in oilfields centrifugal pumps which has two basic components A stationary ring. A rotating ring.

Seal or Packing Packing often is used in low pressure service, or in pumps handling abrasive liquids such as mud or slurries. Packing is composed of a series of pliable rings contained in a packing gland. Mechanical seals generally requires much less maintenance than packing, so they are use whenever possible. When they are use liquid must be free of sand, dirt or other solid particles that can scratch the seal faces and cause leakage.

Couplings The pump shaft connects to the driver with a coupling.

PRINCIPLES OF CENTRIFUGAL PUMPS Liquid enters the pump at the centre or eye of the impeller. Usually impellers rotates at a speed of 1200-3600 rpm. The speed of the impeller creates a centrifugal force that throws the liquid to the outer edge at a high velocity. It leaves the impeller at high velocity and enters the volute, which is enlarged chamber where the velocity is quickly reduced. This velocity reduction results in pressure increase.

PRINCIPLES OF CENTRIFUGAL PUMPS

PRINCIPLES OF CENTRIFUGAL PUMPS The amount of pressure an impeller will develops depends upon its diameter and speed at which it rotates. The large diameter impeller operating at a higher speed will develop a highest pressure. The pressure developed by the impeller is limited by the materials of which the impeller is made. If a single impeller will not develop the pressure required, two or more impellers can be installed in series to increase the pressure rise across the pump. A pump with three impellers can be compared with three pumps which operates in series. There is no theoretical limit to the number of impellers which can be installed in a pump. However, horizontal pumps seldom have more than eight impellers in one casing. If this is not enough to produced a desire pressure, a second pump will be used. Submersible pumps can have 50 or more impellers.

Head Pressure The purpose of the pump is to raise the pressure of the liquid. The amount of pressure rise is called the head pressure or simply head. Head pressure = discharge – suction (pressure)

Head Pressure

HOW CAN YOU DETERMINE YOUR NSPH In an existing system, just read the suction gauge then subtract the vapor pressure. Is that simple “It’s the pressure above vapor pressure”

For a new system being designed, you have to calculated For a new system being designed, you have to calculated. Take the pressure in your suction vessel add the static height of the liquid or subtract it in the case of lift subtract the friction loss on the suction side and then subtract the vapor pressure. Simple!

Cavitation and Vapor lock Cavitation and vapor lock are terms often used interchangeably to describe the pump failure due to presence of vapor in it. Although cavitation and vapor lock both occur when gas is present in a pump, they each have different effects on the operation of the pump.

CAVITATION Cavitation occur when the liquid entering a pump contains a few bubbles of gas. The gas flows through the impeller with the liquid and its pressure is increased in the pump, some or all of the gas liquifies (the vapor bubbles collapse.) A high centripetal force results from this collapse and may cause severe vibration and possible pump damage. The pump will continue to pump liquid, but it will be noisy and may vibrate.

VAPOR LOCK Vapor lock occur when gas enters the pump with liquid and separates from the liquid inside the pump and fills all or a part of the pump. The pump will compress the gas a slight amount, but not nearly enough for the gas to flow out the discharge line. The trapped gas prevents liquid from entering the pump. The effect is that no liquid flows through the pump.

VAPOR LOCK When a pump vapor locks, the discharge pressure gauge reads about the same as suction pressure while the pump is running. In order to clear the condition, the vapor must be removed from the pump. In some cases, this can be done by opening the vent valve while the pump is running. Quite often, the pump must be shutdown and the casing vented until liquid flows out the vent line.

VAPOR LOCK Some pumps are more prone to vapor lock than others. A procedure for starting these pumps is Close a valve in the discharge line. Open valve in suction line. Open casing vent valve until a steady stream of liquid comes out. Start the pump and observe the discharge pressure. It should rapidly increase and then level off. Slowly open the valve in the discharge line. Close the valve in the vent line.

VAPOR LOCK Observe the discharge pressure during stpe-4. If it drops to suction pressure, the pump has vapor lock again, and you will have to shut it down and start over. Cavitation and vapor lock occur when gas is present in the pump. A few gas bubbles will cause cavitation. More will cause vapor lock. This can be done by raising the suction pressure to the pump, or raising the level of the liquid in the vessel that is being pumped.