1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-0a71 Submitted by: Wilson HarkinsSubject: GHe Purging of Hydrogen Systems Practice: Prior to venting a hydrogen (H2) system, initiate a gaseous helium (GHe) sweep purge to evacuate air from the vent line. After venting ope
2、rations are complete, initiate a second GHe sweep purge to evacuate the vent system of residual H2. Use a flapper valve or check valve on the vent line to prevent air intrusion into the line during low or intermittent flow conditions.Programs that Certify Usage: This practice has been used on the Na
3、tional Space Transportation System (NSTS).Implementation Method: This Lesson Learned is based on Maintainability Technique number OPS-04 from NASA Technical Memorandum 4628, Recommended Techniques for Effective Maintainability.This practice greatly reduces the possibility of a vent line fire and/or
4、explosion during H2venting operations. It is impractical to supply the large quantities of GHe required to create a non-flammable H2/He mixture during H2venting operations. The upper flammability limits of a gaseous H2/air mixture is lower with no GHe present. This technique also provides substantia
5、l safety benefits.This technique recommends initiating a GHe sweep purge to evacuate air from a vent line prior to venting a H2system. After the initial venting operation is complete, a second GHe sweep purge should be conducted to evacuate the vent system of residual H2. The upper flammability limi
6、ts of a Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-gaseous H2/air mixture is lower with no GHe present (see Figure 1). A flapper valve or check valve used on the vent line will prevent air intrusion into the line during low or intermittent flow
7、conditions.refer to D descriptionD This practice should be included in all new systems operating procedures and changes initiated to applicable existing procedures. System design should be reviewed to include the following as recommended by NASA TM X-52454 (Lewis Research Center):a71 Include a check
8、 valve/flapper valve or other suitable mechanism to exclude air from vent stacks at low or intermittent flow conditions.a71 Extend vent stacks 15 ft. above a building roof.a71 Discontinue use of ordinary hydrocarbon flame arresters which are incapable of quenching a H2flame.a71 Provide a minimum of
9、a 3-volume exchange (pulse purges) to sweep system prior to introducing hydrogen.Five to 10 volume exchanges to purge a vent system is a commonly acceptable industry practice.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Reference:1. H. Hannah, LSO
10、C 32-30, FCSS Hazardous Commodity Purge Study, dated September 1991Impact of Non-Practice: Use of dilution purges when venting explosive gases such as hydrogen is not necessarily desirable.a71 Mixtures of hydrogen/helium do not become non-flammable until the mixture is 91% He.a71 For “fuel rich“ hyd
11、rogen/helium mixtures in air, the flammability limit increases with increasing He content, until 85% He mixture is obtained.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-0a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
