User Advisory
| Advisory Title: | Interlock Requirements for Turbo Pump Systems on ALS Endstations |
| Advisory Number: | 14/Rev. 2 |
| Issue Date: | January 1, 2005 |
| Review Date: | Note: The advisory procedure and all ALS user advisories are undergoing a complete review that will not be completed until early 2008. Until that time, all current advisories and procedures remain in effect. |
| Current Info: | This Advisory is valid for two years from Issue or Review Date(s), at which time it is reviewed for relevancy and accuracy. Copies of all current user advisories are available at the ALS User Services Office, (510) 486-7745. |
Overview
Oil-lubricated vacuum pumps pose a serious contamination threat to ALS beamlines and potentially to the ALS storage ring. The use of oil-free pumping systems is therefore strongly encouraged for user endstations. If oil-lubricated pumps are used, an effective interlock system must be in operation before the endstation can be opened to the beamline. This User Advisory specifies interlock configurations that are acceptable for turbomolecular (turbo) pump systems on endstations at the ALS. For general information about endstation vacuum policy, see the publications referenced under "More Information" below.
Policy
The ALS vacuum policy for user endstations states, "A turbo pump system (preferably oil-free) with appropriate interlocks may be used in the endstation experimental chamber." Endstation vacuum pumping systems must be configured to prevent beamline vacuum accidents and beamline oil contamination in the event of a pumping failure. This requirement also applies to turbo pumping systems that are intended for roughing use only, since they may contaminate their experiment chambers, and there is no reliable way to assure that the pumping systems are always valved off during beamline operation.
Turbo Pump System Schematic Diagram
The diagram below shows various elements that may be found in turbo pump installations on ALS endstations.
Summary of ALS Turbo Pump Interlock Requirements
| Interlock Element |
Configuration
A |
Configuration
B Oil-free backing pump, oil- or grease-lubricated turbo pump |
Configuration
C |
|---|---|---|---|
| Turbo isolation valve or turbo controller signal to beamline endstation valve (ES VVR1) | Not required | Required | Required |
| Exhaust filter | Not required | Not required | Required |
| Foreline oil trap | Not required | Not required | Required |
| Foreline isolation valve | Optional | Optional | Required |
Acceptable Interlock Configurations
There are three acceptable interlock configurations. These are described below, in order of desirability. For all configurations, the ALS-provided endstation ion gauge (ES IG1) may be read by the user, and it may be relocated to the user's experiment chamber as long as it is not separated from the endstation vacuum valve (ES VVR1) by any other valve.
Configuration A
- Pumping system: Oil- and grease-free turbo pump(s) backed by oil-free backing pump(s).
- Interlock system: No interlock system is required.
- Notes: Configuration A is an inherently oil-free system, the use of which is strongly encouraged by the ALS. Beamline vacuum protection is provided by the existing endstation vacuum valve (ES VVR1) interlocked by the endstation ion gauge (ES IG1). Both are provided on all beamlines. Users who wish to protect experiment vacuum in the event of pump failure may install a foreline valve interlocked to their turbo pump controller.
Configuration B
- Pumping system: Oil- or grease-lubricated turbo pump(s) backed by oil-free backing pump(s).
- Interlock system: With this system, either a turbo isolation valve or the existing endstation vacuum valve (ES VVR1) must be controlled by the turbo pump controller's rpm sensor. (These sensors are usually switch contacts that open when pump rpm falls below a minimum.) A time-delayed vent valve is also required. To use ES VVR1, 24 volts dc must be routed through the controller contacts as an input signal to the beamline's Equipment Protection System (EPS). ALS personnel are available to help implement such a system. (Contact Ken Woolfe at ext. 7739 for assistance.) The time-delayed vent valve is usually mounted on the turbo pump and powered by the pump controller. These valves are normally available from the pump manufacturer. Venting prevents the migration of turbo pump lubricants to the upstream side of the pump when the pump stops under vacuum conditions. The time delay feature is necessary to ensure that the interlocked turbo isolation valve or ES VVR1 is fully closed before venting.
- Notes: A turbo isolation valve is typically large and expensive, but it does protect experiment vacuum in the event of pump failure. If ES VVR1 is used in the interlock system, it will protect only the beamline vacuum. Simple ion gauge control of ES VVR1 is not adequate with oil-lubricated pumps, because a significant quantity of oil can be carried into the experiment chamber before reaching the set point of the ion gauge. Interlocked foreline valves alone are not adequate for use with oil- or grease-lubricated turbo pumps because the lubricant will vaporize under UHV conditions when the foreline valve closes. However, the addition of an interlocked foreline valve or a backing pump power interrupt is advisable to avoid overloading the backing pump in the event of turbo pump failure and subsequent venting.
Configuration C
- Pumping system: Any turbo pump(s) backed by oil-sealed backing pump(s).
- Interlock system: The use of oil-sealed backing pumps is generally discouraged at the ALS. Additional safeguards are required for systems that have oil-sealed backing pumps. All of the interlock features described for configuration B above are required. In addition, an oil trap in the foreline, mounted close to the backing pump, is required to limit oil backstreaming during operation, and an oil vapor filter is required on the backing pump exhaust to eliminate oil discharges into the air or into ALS exhaust ducting. An interlocked foreline valve is required to prevent oil backstreaming under vent conditions.
- Notes: All of the Configuration B notes, above, apply. Please note especially that foreline valves are not adequate for use as main isolation valves for oil-sealed backing pumps. Everything upstream of the foreline valve will be exposed to UHV conditions when the foreline valve closes, and any oil or grease from the turbo pump bearings or previously backstreamed oil in the foreline will vaporize, contaminating the experiment chamber and possibly the beamline.
Multiple-Turbo-Pump Systems
Multiple-turbo-pump systems must also meet the requirements given above. When each turbo pump has its own turbo isolation valve, it may be treated as an independent interlock system; i.e., if one pump fails, its interlocked isolation valve closes to segregate it from the experiment, and the remaining turbo pumps continue to run. Use of turbo isolation valves in multiple-pump systems prevents the loss of vacuum that would occur with only an interlock link to the existing endstation vacuum valve (ES VVR1). In the latter case, the venting of a failed turbo pump gas loads the remaining turbo pumps, slowing them and tripping their interlock systems.
More Information
For additional information about endstation vacuum policy, see ALS User Advisory 9, Vacuum Policy for User Endstations for Protection of Beamline Components and Storage Ring Vacuum, and Advanced Light Source Vacuum Policy and Vacuum Guidelines for Beamlines and Experiment Endstations, LSBL-280 .
For questions concerning ALS vacuum pump interlock requirements, contact William Thur (ext. 5689), or Ken Woolfe (ext. 7739).
| Approved by | Signed |
| Ross Schlueter, Acting Mechanical Engineering Group Leader | 11/20/2004 |
| Ben Feinberg, Division Deputy for Operations | 02/17/2005 |