Whit reference to figure 4, the force exercised by the
spring on the valve can be increased or decreased by
means of the regulation screw under the cap. When the
pressure generated by the pump acts on the lower face
of the valve plate with a force equal to the thrust of
spring, the valve begins to open. The pressure at which
this happens is called the “cracking pressure”. The liquid
begins to flow through the by-pass towards the suction
side. The full flow capacity depends on a pressure which
is a function of the pump rotation speed and the viscos-
ity of the liquid. The cracking pressure is the pressure
setting of the relief valve.
Unless otherwise specified, the by-pass is set at 2 bar.
Full flow through the by-pass is reached at an average over pressure of 15% of the cracking pressure.
The by-pass safety relief valve can be mounted on the pump cover or on top of the casing. In the first case, the regulation screw cover cap
must be on the suction side of the pump while in the second case (stainless steel pumps) it must be on the discharge side.
If the pump is often used in both directions of rotation, it is advisable to mount a double by-pass, available in request, which can be fitted
in the place of a single by-pass.
NB: Do not use the pressure relief valve as a flow regulation device (for example in filling system with a nozzle, in dosage plant, etc). The
liquid which circulates through the pump is subject to heating and this reduces the lubrication of the moving parts, especially of the idler
bush. Furthermore, the liquid may vaporize or from incrustations. In these cases, a return to tank line with a spring valve or other type of
valve must be fitted to the pipework, or a system to regulate the pump rotation speed installed. The pressure relief valve should therefore
be used only as a safety valve to prevent damage to the pump or pipework due to exess discharge pressure.
If it is not possible to fit a relief valve, a system which breaks the power transmission between the motor and pump, such as a torque break-
er coupling. Torque converters are not suitable for this purpose; they only ensure smooth start up of the pump. AT and AW model pumps
for abrasive liquids cannot mount a by-pass because they are not suitable for products which wear out the valve seat.
7.5
The base plate on which the unit is mounted must be sufficiently robust and should absorb vibrations rather than cause or amplify them.
Steel base plates supplied by Varisco meet these requirements. The foundation on which the base plate is to be mounted should be per-
fectly level and at least 50-100 mm wider than the base plate.
To advoid deforming the base plate, shims should be added to the anchor bolts as necessary. Anchor bolts, whose shapes are specified
by sized in proportion to the holes on the base plate.
Check the alignment of the unit after tightening the anchor nuts and before connecting the piping.
7.6
The pipework must be suitably sized to ensure that the pump operates with the required performance. Most problems are due to the suc-
tion line. Other sections of this manual contain information for sizing the pipework. The following are some of the most important points:
7.6.
1
The diameter of the pipes must be less than that of the pump ports.
7.6.
2
Ensure that the pipes are clean and do not contain foreign bodies such as nuts, screws, rags, welding slag, pieces of electrodes, etc.
7.6.
3
Mount a non return valve in the piping to avoid having to fill it every time the pump is started. If the pump operates on a suction lift, install
a foot valve which is large enough not to create excessive friction losses.
7.6.
4
The suction pipe must have a diameter equal to or greater than that of the pump suction port and should not have upward bends where
air pockets can form.
7.6.
5
The joints of the suction pipe must be perfectly airtight: use flanged joints or threaded joints with teflon tape or tow on the threads. During
pumping, the pressure in the suction pipe is lower than atmospheric if the pump operates on suction lift. This means that any leaks in the
pipe will not cause liquid to drip from the pipe but air will be drawn in through the leaks which is more difficult to detect.
7.6.
6
It is recommended that pressure and vacuum gauges be installed to check the pressure. Most V pump models have connections near the
suction and discharge flanges for mounting gauges.
7.6.
7
Install a filter in the suction line to intercept any impurities or foreign bodies in the liquid which could damage the pump. The filter basket
must have a free passage surface losses on the suction side of the pump. Special care should be taken when filtering high viscosity liq-
uids: it is better to oversize the filter. The filter screen holes must be sized so as not to impede the operation of the pump. The following
table shows the maximum size of the filter screen holes for the various pump models:
The filter should be installed in such a way that it is easy to clean. Take care during
installation that the direction of flow is that indicated by the manifacturer. The liquid
should always flow from the inside towards the outside of the basket.
Varisco makes filters with replaceable basket which can be opened without tools
(figure 5). The drawing shows the reccommended mounting position in which the
filter basket can be quickly cleaned.
lock nut
regolation screw
cap
regolation screw
cap
6
fig.4
suction pipe
pump
fig.5
V20, V25, V25-2, V30-2, V50-3, V60-2, V70-2, V80-2, V85-2,
V90-2, V100-2
V120-2, V150-2, V151, V180, V200
0,8
1
Max size (mm)
Pump size
