8.4
It is always advisable to mount a by-pass relief safety valve which can be supplied and mounted even after the pump has been installed.
If for any reason the discharge line should be blocked when the pump is operating, the pressure generated can reach extremely high levels,
thus endangering the transmission organs or the pipework joints. The relief valve is mounted externally on the pump cover or casing and
can be calibrated from the outside.
With reference to figure 4, the force exerted by the
spring onto the valve can be either increased or
decreased by means of the regulation screw positioned
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 the spring, the valve will
begin to open. The pressure at which this occurs is
called
“
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 viscosity 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 onto 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 whilst in the second case (stainless steel pumps), it must be positioned on the discharge
side.
If the pump is frequently used in both directions of rotation, it is advisable to mount a double by-pass, available upon request, which can
be fitted in the place of the single by-pass.
ATTENTION:
Do not use the pressure relief valve as a flow regulation device (for example in filling systems with a nozzle, in dosing plants
etc.). The liquid which circulates through the pump is subject to heating and this causes the reduction in lubrication of the moving parts,
especially of the idler bush. Furthermore, the liquid may vaporise or cause incrustations. In these cases, a return to tank line with a spring
valve or other type of valve must be fitted to the pipework otherwise, a system to regulate the pump rotation speed may be installed. The
pressure relief valve should, therefore, be used only as a safety valve in order to prevent damage to the pump or pipework caused by
excessive 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
breaker coupling should be fitted. Torque converters are unsuitable for the following reason; they only ensure smooth start-up of the pump.
AT and AW model pumps for abrasive liquids cannot mount a by-pass as they are not suitable for products which wear out the valve seat.
8.5
The base plate onto which the unit is mounted should be sufficiently robust and should absorb vibrations rather than cause or amplify them.
Steel base plates supplied by Varisco meet these requirements. The foundations onto which these base plates are to be mounted should
be perfectly level and at least 50-100 mm wider than the base plate. In order to avoid deforming the base plate, shims should be added to
the anchor bolts if required. The anchor bolts, with forms that are defined in the current normative and available upon request, should be
adequate for the force applied and must have a diameter which is proportional to the holes of the base plate.
Check the alignment of the unit after tightening the anchor nuts and before connecting the piping.
8.6
The pipework must be suitably sized to ensure that the pump operates with the required performance. Most of the problems are related to
the suction line. Other sections of this manual contain useful tables related to the correct dimensions of the system. It is, however,
necessary to follow some important points:
8.6.
1
The diameter of the pipes must never be less than that of the pump ports.
8.6.
2
Ensure that the pipes are clean and do not contain any foreign material such as nuts, screws, rags, welding slag, pieces of electrodes, etc.
8.6.
3
Mount a non-return valve in the piping in order 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 losses due to friction.
8.6.
4
The suction pipe must have a diameter equal to or greater than that of the pump suction port and should not contain any upward bends
where air pocket can be formed.
8.6.
5
The joints of the suction pipe must be perfectly airtight: use either flanged or
threaded joints with teflon tape or tow on the threading. During pumping, the
pressure in the suction pipe is lower than the atmospheric pressure if the pump
operates on suction lift. This means that any leaks in the pipe will not cause the
liquid to drip from the pipe but air will be drawn in through the leaks which is more
difficult to detect.
8.6.
6
It is recommended that both pressure and vacuum gauges be installed to check the
pressure. The pumps are fitted with connections adjacent the suction and discharge
flange for the mounting of gauges.
8.6.
7
Install a filter in the suction line in order to intercept any impurities or foreign bodies
in the liquid which could damage the pump. The filter basket must have a surface for
free passage on the suction side of the pump. Special care should be taken when
filtering high viscosity liquid; it is recommended to oversize the filter. The filter screen
holes must be sized so as not to impede the operation of the pump. The following
table indicates 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. During installation,
take care that the direction of flow is that indicated by the manufacturer. The liquid
should always flow from the inside of the basket towards the outside.
Varisco manufactures filters with replaceable baskets which can be opened without
the aid of tools (fig 5). The drawing indicates the recommended mounting position
in which the filter basket can be cleaned quickly.
lock nut
adjustable screw
cap
adjustable screw
cap
6
fig.4
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
Dim. (mm)
Pump type
fig.5
to the aspiration tubing
to the
pump
