The pump SUCTION line size is typically kept one standard size larger than the pump DISHARGE line in order to avoid suction NPSH and suction problems. The headloss from fluid flow can be excessive if the pipes are too small.

More information required - your question doesn't make much sense.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)  

RS82,
Your posting (Why is the suction line one size smaller than discharge?) is incorrect.
Typically (on an end suction centrifugal pump):
a). The Pump Suction line is one (or more)sizes larger than the pump suction nozzle.  
b). The suction nozzle is one size larger than the discharge nozzle.  
c). The discharge line is normally one size larger than the discharge nozzle.
Example:
Suction line size = 8" or 10"
Pump suction nozzle size = 6"
Pump discharge nozzle size = 4"
Discharge line size = 6"

You should also read this posting
http://www.eng-tips.com/viewthread.cfm?qid=239699

In the future you should also use the "Search" (located at the top of the page) to see if the question has already been asked.

The line size should roughly agree with the velocity at the inlet and outlet of the pump at BEP conditions.  Since pumps accelerate the fluid between inlet and outlet, the outlet size has a higher velocity, but flowrate is the same, giving a smaller area and  diameter.  

From "BigInch's Extremely simple theory of everything."

Sorry for the typo folks..the suction line is larger than the discharge..understood as they want to minimize losses in the suction which increases NPSHa..but what the reason behind having the discharge less then suction?  Are there any instances where the size of the lines are the same?

Thanks for your help!

The reason is not directly related to NPSH, although a larger suction diameter would obviously help.  

Please read the previous post.

From "BigInch's Extremely simple theory of everything."

RS82,

It all depends on the application.

For short distances on water piping such as in a building, 7-8 ft/sec pipe velocity is economical on the pump discharge. If you are pumping a long distance, 3-5 ft/sec pipe velocity is appropriate on the pump discharge.

You may have other considerations such as solids in the fluid, fluid properties such as viscosity, fluid velocity, pipe materials available on the jobsite, etc.

The rule of thumb for suction piping is "least one size larger than the suction flange".

Suction piping should be at least one size larger than the suction flange at the pump.




 

Suction and discharge piping need to be what they need to be and can connect to whatever inlet and outlet size of the pump happens to be, but if you look at the inlet and outlet of the pump I always see the inlet is at least one size larger than the discharge.

From "BigInch's Extremely simple theory of everything."

Something this and all other original posters (OP) need to understand is the hazards of vagueness and gross generalizations.

He or she did not define:
 - the type of pump (Centrifugal or Positive Displacement)
 - the configuration of the pump (end suction/top discharge, top suction/top discharge, side suction/side discharge, single stage, multistage, Base plate mounted, In-Line, etc.)
 - the size of the piping and pump nozzles
 - the commodity
 - the type of installation (refinery , power, pharmaceutical, Food, Central plant HVAC, etc.)
 - the country where installed

The better the information supplied the better the answer given.  

Sorry this was not pertaining to any particular job hence the lack of information.  It was a more of a generic question as my mananger had stumped me on the question of why cant the discharge nozzle and suction nozzle be the same?  Why Suction one size larger..this was his question and he left me looking like a deer looking in a headlight..I am novice when it comes to pumps..thanks everybody that replied

RS82,
To answer your question, yes in certain cases a suction line and discharge line may be the same size, as noted in this string of comments, generally the suction line is bigger than the discharge line for NPSH and velocity issues, however, depending on the type of pump you are referencing this may change.
There are a number of pump manufactures that offer HSC pumps with the same size on both discharge and suction.
Also most all your vertical turbine centrifugal pumps are with the same size column (suction) as the discharge.
So there are many factors, along with the type and make of pump, to determine whether the pump may be supplied with the same size suction and discharge.
hope this helps.