For delivery pipes of considerable length (therefore for high power pumping systems), the diameter is chosen according both technical and economic criteria.
It is advisable for the delivery pipe to be provided with a check valve and with a regulating valve. The former, which should be upstream from the latter so that it may be inspected and replaced if necessary without emptying the delivery pipe, must protect the pump against water hammer and prevent inverse flow though the impeller in the event of sudden stopping of the unit. The regulating valve, on the other hand, is used to vary the power raised by the machine and therefore to satisfy the requirements of the user.
 

2) The suction pipe requires greater attention than the delivery pipe, as a mistake in its installation or in choosing its dimensions may cause serious problems (the major one being cavitation, with all the problems that accompany it).
To guarantee the best and most efficient suction conditions, this pipe must be as short as possible, vertical or rising towards the pump (so as to avoid air pockets which could cause the pump to become unprimed), without choking or sudden changes in direction.
Save in the exceptional case of complex systems, along the suction pipe there must not be valves of any kind except the foot valve fitted at the immersed end of the suction pipe; in the case of Jet self-priming pumps, the foot valve may be replaced by a check valve fitted directly on the suction mouth.
It must also be remembered that, in order to guarantee suction of the liquid to be pumped, this pipe must be hermetically closed, especially if there is a high geodetic difference in level at suction. At the same time there must be no high points where the air and/or gases dissolved in liquid can gather, thus causing an interruption in the flow; it is therefore best to fit flanged or threaded couplings, discarding solutions that consider cup or bell couplings which frequently cause an imperfect seal.
To avoid the formation of vortices, and the consequent intake of air with the liquid to be pumped, the input section of the suction pipe must be about 0.4 - 1 m below the free suction surface, depending on the dimensions of the system.
For short suction pipes the flow rate and capacity values indicated above may be chosen, multiplied by 0.8; however, the internal diameter must never be smaller than the diameter of the pump suction mouth.

3) Each pump should, if possible, have its own suction pipe so that it may be easily excluded from the system if the need arises, and also to prevent the entry of air in the suction manifold when the pump is stopped. If a single suction pipe has to be designed for several pipes, the delivery check valves must be eliminated and fitted on the suction mouths instead.

4) Special pieces (such as curves, elbows, derivations and connections, etc.) and various valves should be used only if built suitably: for further information on this subject, consult a good book on hydraulics or on pumping systems. In all cases remember that:

1. the connections must be accurate so to avoid air re-entry and to prevent the gaskets from reducing the working section of the pipe passage, increasing its resistance;
    
2. to choke the capacity, regulate only the gate valves fitted on the delivery pipe and never those on suction.