24

1 General Pump Selection

1.13

Concept of wet-instal-

led pumps

The most straightforward

concept for a pumping station is

the installation of wet-installed

pumps. The pumps are installed

directly in the suction chamber

and completely or partly

surrounded by waste water

during operation. The pumps

can either be operated with a

vertical shaft driven by a motor

mounted above the fluid level,

or the entire pump sets can be

used under water as submersible

motor pumps.

In practice, wet-installed pumps

were mainly used in smaller

pumping stations (flow rates up

to approximately 1,600 qpm);

they are called packaged pump

stations [1.17; 1.18].

The benefits of a wet instal-

lation are also increasingly

employed in large pumping

stations (flow rates up to ap-

proximately 255,000 qpm). The

advantages and disadvantages

of this arrangement are listed in

Table 1.1 [1.19].

To prevent clogging,

narrowing the pipes in the

direction of flow must

absolutely be avoided. The

inside diameter of the discharge

lines must be equal to or greater

than the inside diameter of the

discharge nozzle [1.20]. Use

pipe con nections without

narrowing and make sure that

open valves do not obstruct the

flow [1.21; 1.22]. Calculate the

optimum diameter based on the

mini mum flow velocity and use

the next largest pipe diameter

available. For longer discharge

lines measures to reduce surge

pressure have priority over

measures to maintain the

minimum flow velocity. For

discharge line lengths of up to

1,640 ft a maximum velocity of

2,5 m/s (8.2 ft/s) applies [1.23].

The piping material must be

suitable for the waste water to

be transported. Make sure it is

corrosion-resistant (hydrogen

sulphide!). This also applies to

the material of the pipe

supports, which need to be

closely spaced. The piping must

be fastened without

transmitting any forces to the

pump. The pipes should have a

smooth inner surface, be able to

withstand cyclical loads and be

selected, depending on the

discharge head, for an

operating pressure of 87 to 145

psi. The discharge line from the

pump must be connected to the

main line with a horizontal

section. Avoid abrupt changes

of direction. Pipe connections and

fittings must comply with the

relevant product standards.

Butterfly valves are unsuitable as

isolating valves in the waste water

sector (see overview in Fig.

4.2.3a). We recommend soft-

seated isolating gate valves with

external thread, body made of

GGG (ductile iron), stem made of

1.4571(stainless steel) , stem nut

and seat rings made of 2.1060.

Choose actuators whose

maximum actuation force cannot

damage the gate valve [1.24].

Especially suitable as non-return

valves are swing check valves with

lever and weight. The body

material we recom mend is

lamellar graphite cast iron (GGL)

or nodular cast iron (GGG)

[1.25]. For low flow velocities,

non-return ball valves can be used

[1.26].

The non-return valve should

be arranged vertically and as high

as possible above the pump to

allow the rising fluid level to

release the air it displaces in the

pump. If this condition is met, a

venting device for the pump is not

required [1.27].

according to ISO CuSn12Ni2

according to ASTM (USA) C91700

GGG nodular cast iron

GGL lamellar cast iron

1.4517 Stainless steel

2.1060 tin bronze