Peter Skop Industries

Compressed Gas Division

Sherwood® Valve Installation Guidelines

As a Stocking Distributor, PSI can quickly ship a large selection of Sherwood Compressed Gas Products from our inventory. 

Valve Installation Guidelines

and

General Information for Sherwood

NGT Cylinder Valves

 

The information in this document does not necessarily represent the opinions of other valve manufactures, nor is it necessarily applicable to their valves.  Neither is it to be construed that the valve installation guidelines herein represent the only acceptable means for installing valves into cylinders To accomplish proper leak tight connections it is essential that the valve and cylinder threads be free of burrs and damaged threads and to be within thread specifications.  The latter can be ascertained only through the use of the correct, calibrated thread gauges.

 

1. For new brass valves in new steel cylinders, CGA Standard V-1 and Federal Standard H-28 provide for a hand tight engagement (4 ¾ turns, nominal) plus wrenching (3 turns) for standard ¾” –14 NGT threads.

2. For other thread sizes, hand tight engagement is the applicable value in inches in the second column of Table 3 of CGA Standard V-1, see attached, multiplied by the “threads per inch” which is the number preceding “NGT” in the thread designation: e.g. for a ¾”-14 NGT thread, the nominal hand tight engagement equals .339 x 14= 4.746 or 4 ¾ turns.

3. Hand tight engagement is what a person can apply with gloved hands on a bare metal-to-metal connection.  The difference from person to person, regardless of size and sex, will not significantly differ provided the person exerts his or her maximum torque.

4. Hand tight engagement occurs when the first full thread on the valve engages the thread in the cylinder with the same pitch diameter; i.e., when it finds its mate.

5. To compensate for luting compound, Teflon tape, etc., hand tight engagement should first be determined under bare metal-to-metal conditions; then determined after the luting compound and/or Teflon tape has been applied.  The amount the second torquing varies for the first may be added or subtracted form the wrenching turns to compensate for the tape or lubrication.

6. There are no clear guidelines the standards for recommended wrenching turns as a function of thread size; nor is there a generally accepted recommendation for wrenching turns as a function of torque.  It is common practice to wrench a ¾” –14 NGT new brass valve into a new steel cylinder about 3 turns after hand tight engagement.  This requires about 250-300 ft-lb of torque; but torque is not always a dependable criterion to ascertain a leak tight seal.  Torque can be generated quickly if the lead thread on the valve is damaged or, if valve and/or cylinder thread quality is not up to standard.

7. For NGT threads larger than ¾”, the criteria in (6.) has also worked satisfactorily, i.e., 3 wrench turns.  For sizes smaller than ¾” somewhat fewer wrenched turns may provide a leak tight seal.  For example, ½”-14 NGT thread wrenched 2 to 2 ½ turns may be sufficient.  In the extreme, a 1/8” NGT thread such as for certain fuse plugs, need be wrenched only 1 ½ turns after “finger tight” engagement.

8. With reference to Figure 1 in Section E of CGA Standard V-1, L1 refers to hand tight engagement; L3 refers to wrenched turns; and L9 refers to the length of full threads provided in the cylinder.

9. The total number of full threads on the valve is equal to the number of threads per inch multiplied by the value in the L8 column of Table 3 in V-1.  Note that full threads are threads with full roots.

10. Hand tight engagement can vary from nominal by +/- 1 turn tolerance on both the valve and the cylinder.  Thus, for a ¾” NGT thread, hand tight engagement can vary from 2 ¾ to 6 ¾ turns.  This four-turn difference is why counting exposed turns after wrenching is not a good indication whether a wrench tight seal has been accomplished.

11. The maximum bore diameter column in Table 3 is important because if the bore size in the cylinder prior to tapping is exceeded, the threads at depth L1 + L3 and greater in the cylinder may have flat crests right where the valve is trying to create a seal.  See Figure 1.

12. As shown in Figure 2 and as indicated in (4.) above, hand tight engagement, L1, is obtained when the pitch diameter of the valve’s first thread mates with a thread in the cylinder of the same size.

13. To ensure the initiation of a seal is at the lower end of the valve, the taper tolerances on the valve and cylinder are unilateral and in opposite directions.  As shown in Figure 2, the taper tolerance on the valve is plus no turns (0) minus one (1) turn, i.e., a “slow” taper.  The taper tolerance on the cylinder thread is plus one (1) turn minus no turns (0), i.e., a ”fast” taper.

14. Figure 2 titled “Valve Inlet Design” was added to CGA Standard V-1 to control the size of the valve inlet diameter and propulsion effects should the valve be severed at the top of the cylinder neck.  For service pressures over 500 psig, this hole should not exceed .300” in diameter beyond the minimum length of L1 + L3 which for a ¾” – 14 NGT valve is 2 ¾ + 3 turns or 5.75 turns which is equivalent to .411”.  Exceptions where a larger diameter is necessary are identified in the notes in Figure 2.

15. There is much confusion as to the difference NGT, NGT (Cl), and NPT threads.  All three threads are identical relative to the size of the first thread and nominal thread taper (3/4 inch per foot).  They differ relative to taper tolerances on NGT and NGT (Cl) valve and cylinder threads are plus zero turns minus one turn and plus one minus zero turns, respectively; whereas the taper tolerance on an NPT thread is plus or minus ½ turn.  The thread lengths are also different; the NPT thread being the shortest and the NGT (Cl) thread the longest.  For example, for ¾ “ threads, the full thread lengths are as follows:

 

NPT                        .5457”               (7.64 threads)

NGT                        .7676”               (10.75 threads)

NGT (Cl)   .9461”               (13.25 threads)

NGT threads are longer than NPT threads to increase valve life.  NGT (Cl) threads are the longest because it has been a common practice in the chlorine industry to remove and recondition valves prior to each refilling operation.  This results in accelerated wear.  To ensure maximum cylinder life, oversize valves are authorized.  The “Cl” means “chlorine” not “class”.  (Cl)-1,-2,-3,-4, and –5 refer to 0, 4, 8 ½, 14, and 28 threads oversize so as to provide valves that will fit in worn cylinders of varying sizes.  As can be seen from Table 3 in CGA Standard V-1 there are no specifications for oversize cylinder threads because cylinders that accept valves manufactured to these classes are ‘worn” cylinders with no standards.

16. Oversize threads are sometimes specified for both new valves and new cylinders by some users so that their valve/cylinder assemblies are unique or, at least, identifiable as their product.  In such cases, these threads are not given the “Cl” classification unless the size matches a specific chlorine valve size.

The important thing to remember about oversize threads is that an “n” oversize NGT thread is a thread whose first thread is 0.004465 x “n” larger than the first thread on a basic NGT thread.  Thus, the first thread on a four thread oversize valve is the same size as the fourth thread on a basic NGT thread.

Note:  ¾” taper per foot, or 1/16” taper per inch, divided by 14 threads per inch equals 0.004465” which is the amount each thread is larger or smaller in diameter than the thread next to it.

To demonstrate this, note in Table 3 that the major diameter D_o at the small end of the valve (first thread) for (Cl)-2 four thread oversize valve is 0.0179” larger than the first  thread of a (Cl)-1 basic valve or 4 x 0.004465”.

17. When tapered threads are wrenched beyond hand tight (L1) engagement, a negative clearance is generated and something has to give; usually the threads on the lower end of the valve are deformed.  They are forced “out of tolerance” and for that reason, a valve that has been removed from a cylinder cannot be meaningfully gauged thereafter.  In fact, if gauging is attempted, one could damage the gauge.  This does not preclude reuse of the valve.  When reused, the valve will insert deeper into the cylinder and use of previously unused threads will occur.

18. If re-engaged in a used cylinder at the same torque, a used valve normally engages about one thread deeper than its previous insertion.

19. The 250-300 ft-lb installation torque for brass valves in steel cylinders may cause galling and seizing of stainless steel valves in the same cylinders.  To avoid this and still achieve a leak tight connection, less than 200 ft-lb on a new stainless valve may be sufficient.

20. For aluminum cylinders the cylinder manufacturer should be contacted for proper valve installation procedures.