© Edwards Limited 2007. All rights reserved. Page 21
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Installation
B344-21-880 Issue N
3.4.2 Backing and roughing connections
Ensure that the connecting pipeline to the backing pump is as short as possible and has the maximum possible internal
diameter.
You must use the correct type of exhaust extraction system for your process. The extraction system must be designed
to withstand the pressure of operation and, when hazardous materials are produced or processed, must be
sufficiently leaktight to contain the process materials and their by-products.
On all applications, we recommend that you fit a backing pressure-interlock to the backing pipeline, to switch off
the electrical supply to the pump heater(s) if the pressure in the pipeline rises to the critical backing pressure. You
must fit a backing pressure-interlock if you will pump dangerous substances. If you do not fit a pressure-interlock and
the pressure in the backing pipeline rises to the critical backing pressure, fluid will backstream into the vacuum
system and thermal breakdown of the pump fluid may occur. Note that if you have a suitable backing pressure gauge,
you may be able to use the gauge to provide the necessary pressure signal for the pressure-interlock.
The components required to connect the backing and roughing ports on the Diffstak are shown in Figures 12 and 13.
Some of these components are supplied with the pump (refer to the footnotes on Figures 12 and 13); other
components can be obtained from Edwards (refer to Section 7) or from alternative suppliers.
We recommend that you use a Edwards combined roughing/backing-valve Model BRV as it is designed especially for
the Diffstak 63/150, 100/300 and 160/700 pumps. You will have to use two separate valves (such as Edwards
Model PV40) for selection of the roughing or backing mode of operation when you couple a Diffstak 250/2000 pump
into your vacuum system.
Use the following procedure to connect the backing-flange to the backing pipeline.
1. Ensure that the backing-flange and backing pipeline sealing faces are clean and scratch-free; refinish the sealing
faces if necessary.
2. Apply a light wipe of pump fluid or vacuum grease to the ‘O’ ring supplied.
3. Use a trapped ‘O’ ring and NW clamping ring (not supplied) to connect the backing-flange to the backing
pipeline.
Conduct the exhaust to a suitable treatment plant to prevent the discharge of dangerous gases and
vapours to the surrounding atmosphere.
If you will pump dangerous substances, you must fit a backing pressure-interlock.
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