WK
9.2
Isolating Valves
An isolating valve (gate valve) should be provided in the suction
lift line, to enable the supply of fluid to a pump to be shut off if
necessary. An isolating valve should also be incorporated in
the discharge line of every pump, as close as possible to the
pump itself. This valve can be used to adjust the operating
point (rate of flow) apart from its function of isolating the
discharge line. Isolating valves in suction head lines should
only be used to isolate the line (in the event of repairs etc.).
They must always remain fully open when the pump is running.
If the pump operates under vacuum or suction lift, the isolating
valve should be provided with a sealing liquid connection or
with a closed water seal, to prevent any ingress of air into the
stuffing box of the valve stem. To facilitate venting the isolating
valves should be fitted in the line with their stems horizontal.
9.3
Non-Return Valves (in the discharge line)
A check valve or non-return valve should be incorporated
between the pump and isolating valve. Depending on the
circumstances, this can be either a check valve, or a non return
valve or an automatic recirculation valve. The object ofthe non
return valve is to prevent a reflus of fluid through the pump
when the latter stops suddenly. A blocked or leaky non return
valve may cause the pump to rotate in reverse, slackening the
shaft protection sleeves and damaging the pump.
9.3.1 Automatic Recirculation Valve
The schroeder system automatic recirculation valve (minimum
flow device) is a safety device, the purpose of which has already
been explained in section 2 ‘Mode of Operation of Pump’. It
should always be installed immediately downstream of the
pump, always upstream of the isolating valve, and always
vertical, with the direction of flow from bottom to top (see Fig.
48).
Each automatic recirculation valve is supplied in accordance
with the operating conditions of the pump concerned.
The greater the flow of fluid, the higher the valve cone is lifted
by the fluid pumped. A connecting rod in the shape of a lever
the slide valve lever is connected at one end to the valve cone
and at the other end to the shut-off valve (slide valve) on the
bypass (leak-off) outlet. As the valve cone rises and falls, the
shut-off valve is actuated by this lever, and the opening of the
bypass is controlled in such a way that the bypass closes when
the rate of flow has attained a given value, and opens when it
drops below this value. The minimum flow rate is calculated
and adjusted so as to avoid any excessive overheating inside
the pump (see Fig. 49).
Part No.
Designation
1
Bottom half of body
2
Top half of body
3
Valve cone
6
Guide shank
8
Slide valve head
9
Nozzle
10
Throttle
11
Rotary slide valve
1. Strainer holder
2. Fine strainer
3. Coarse strainer
4. Pump
Fig. 46 Conical strainer
for suction head line
Fig. 47 Conical strainer with
monitoring of pressure drop.
1
2
3
4
min.
max.
Fig. 48 Automatic recirculation valve
2
15
6
18
16
21
3
8
19
11
10
20
9
23
22
13
12
17
14
1
24 25
‘‘Construction with manual
operation nozzle’’
Fig. 49
12
