Pumps

A.     Pump

a pump is a mechanical device used for adding energy to a liquid passing through it. A pump may raise the liquid, or force it into a pressure vessel or merely give it enough head to overcome pipe friction.

B.      Turbine

a turbine is a device which utilizes the energy of a current of fluid, acting on a series of curved veins, to rotate a central shaft or spindle. This mechanical energy is in utilize to drive pumps, compressors and generators.

C.      Types

The three major types of pumps are;

A.      Centrifugal

These are the most common in our industry and calm prize the majority of pumping applications. The radial flow centrifugal is the most common type of centrifugal one in which the pressure is developed principally by the action of centrifugal force. The liquid enters the pump impeller at the hub and flows radially to the periphery.

B.      Reciprocating

These pumps consist of a piston or plunger and displace a liquid and force it out of the discharge nozzle. Both suction and discharge piping are subject to pulsation in this type of pump.

C.      Rotary

A positive displacement pump consisting of a chamber containing cams in which rotate relative to the driveshaft and displace liquid strictly by mechanical means.

Location and arrangement

Pumps, regardless of type, should in general be located and arranged with consideration to the following;

A.      Locate as close as possible to the suction source, keeping the suction piping as short as possible, and minimizing fittings insofar as practicable from a flexibility standpoint.

B.      Arrange multiple pump units to provide ample access space for in-place maintenance and service.

C.      Arrange pumps so that the suction piping can be routed without pockets

D.      Pumps at storage tanks should be located outside of the dikes area

E.       Pumps operating at or above their auto ignition temperature should not be located below pipe racks

F.       Provide sufficient support said suction and discharge piping to prevent excessive load and forces on pump casings

G.     Location and arrangement of sump pump, and design criteria for sizing pumps is covered in detail in the hydraulic Institute standards under centrifugal pump applications.

H.       Locate pumps to satisfy the NPSH requirements of the pumps. Net positive suction head or NPSH is defined as the total suction head in feet of liquid absolute determined at the suction nozzle and corrected less the vapor pressure of the liquid in feet absolute.

Taking the phrase net positive suction head word by word may help said some light on it.

Head – one means of indicating pressure, usually in feet of liquid being pumped.

Suction head – head at the suction port of the pump.

Positive suction head – positive or plus, as opposed to negative or minus, means that the suction head must always be a positive term. Since many applications involve the vacuum at the pump (negative gauge pressure), we must always use the absolute pressure scale. Perfect vacuum equals zero absolute pressure this assures a positive value.

 

Net positive suction head – net, here as with your income, means what is left after a gross amount of certain deductions or additions. In the case of NPSH the grosses the absolute pressure at the service of the liquid in the supply tank. One deduction is the liquid vapor pressure; another is a pressure drop due to pipe friction losses in the suction line; their deduction is the static suction lift (the vertical distance in feet); if there is a static suction head it is in addition. After these deductions additions, we are left with the net positive suction head.

 

Why all the fuss about NPSH available? Because: all pumps have an NPSH requirement. All pumps have, as a result of their particular principal and design, an internal pressure drop from their suction port into their pumping elements.

 

If the pressure provided at the suction port by the system (NPSH) does not equal or exceed this internal pressure drop, the liquid at the pump will flash (pump will cavitate). The pressure necessary at the pump suction port to prevent the pump cavitating is called the net positive suction head.

 

 

The process engineer fills out the  NPSH available entry when specifying the pump. He or She may consult with a layout designer for height of suction vessel or piping configuration in cases where engineering feels vessel elevation is critical.

The height of the supply vessel above the pump suction and the friction loss of suction piping are the primary concern of the layout designer with regard to NPSH. Where arrangements of pumps and vessels fail to satisfy NPSH requirements or appear impractical or unsatisfactory, consult with the process or equipment engineer to see if a different type of pump or other solutions may be found.  

 

 

Piping

Pipe routing is critical to efficient pump operation. Piping to pumps and their turbine drivers should be carefully designed to ensure trouble-free operation.

 

Suction piping guides are listed as follows:

A.      Route suction lines without pockets. Some piping arrangements appear to be trouble-free but can cause problems when in operation.

B.      Any reduction in line sizing must occur as close as possible to the pump suction nozzle

C.      Temporary suction strainers must be used at all pump sections. These may be flat plate or conical type. Provision must be made for removal of the strainers.

D.      Suction headers with dead legs must be avoided

E.       Suction piping must be routed to allow for thermal expansion. Pumps with spares cause problems and high temperature applications because the one running pump is hot with a spare is cold. Sufficient supports must be provided to prevent excessive loans from being transmitted to pump casings.

F.       Valve stems on both suction and discharge line should be carefully oriented to prevent becoming a hazard to operating personnel. Locate away from head or eye level.

G.     Overhead horizontal lines to and from pumps should be kept out of the area directly over the pump, if at an elevation where they could obstruct pump removal.

H.      Discharge piping should always be provided with a check valve preceding the block valve. Check to types of check valve for position requirements. Some types of check valves must be installed in a specific position to operate correctly.

I.        Ample flexibility in discharge piping is a must. Provide sufficient supports

Access maintenance and safety

Insufficient space around pumps for access and maintenance is a common problem in process plants. This is caused by the fact that many times equipment is located using preliminary information regarding pump sizes, and by the designers not realizing the relative size of piping and equipment. 1 m clear space between adjacent piping is an absolute minimum. When locating pumps using preliminary pump sizes the designer must allow ample space between pumps. It is better to have too much space than not enough.

Pumps inherently require frequent attention and service. Maintenance aisles must be carefully planned and piping sufficiently supported to enable dismantling of portions of piping for removal of unit without the need for extensive shoring or temporary supports.

 

From a safety standpoint, as mentioned previously, pumps operating at or above their autoignition temperature must not be located beneath pipe racks.