Required Torque

I don't know much about seals at those pressures and temperatures but do know just enough practical hands on knowledge about hydraulic systems to feel that his 5000 -6000 ft-lbs is probably about 10 times too much for that size of plug.

Over tightening is often times worse than under tightening. If its under tight you at least have the chance that tightening it a little more will stop the leak. Over tight and still leaking means you have thread and seal damage and probably metal warping and fatigue already present and the parts as a whole are far more likely to have a massive failure.

In hydraulics over tightened fittings tend to have much more problems with catastrophic failures than a slightly loose or properly torqued fitting does.

If it gets twisted off or striped out while doing it his way make him do the repair work himself. If it blows out while in use make him do the repair work and damage control as well.

See MIL-HDBK-60 for a starting point.

A 4" diameter bolt will withstand at least 5000 lb-ft torque. As you can see from this chart, a grade 5, 1.5" bolt is torqued to around 2000 lb-ft. You can also see that doubling the diameter from 3/4" to 1.5" changes the torque by a factor of about 8. Following this logic, a 4" bolt would be torqued to more than 5000 lb-ft.

Of course, the tension on the bolt depends upon thread pitch, surface finish, lubrication (or absence of same) etc. The allowable tension depends on the alloy, heat treatment, etc. When working with something of this size, I'd want to contact the manufacturer of the vessel and or fittings to get a feel for appropriate torque. 100 - 200 lb-ft seems extremely low. (Car wheel lug nuts are often incorrectly torqued to over 100 lb-ft, by tire changers who don't care. Car wheel studs have about 1/64 the area of a 4" fastener). There can be something about the design of the fitting or flange that does not permit full torque to be applied to the fastener (e.g. is there a taper involved?).

Often, big nuts and bolts are "torqued" with a slugging wrench and sledge hammer, which results in a torque of 1000 lb-ft (on a 1.5" - 2" bolt) pretty easily.

But in any case, my advice would be to avoid guessing and consult the manufacturer of the pieces involved. Anything operating at 30,000 psi pressure is not a toy.

Your torque numbers are reasonable for a 4 inch bolt but he has a 4 inch pressure fitting.

I have no problems torqueing a 1 inch bolt down with a few hundred foot pounds but I wont torque a 1 inch hydraulic fitting or pipe fitting to anything near that!

You're right. I think I completely misread what he was doing!

A threaded cap on a 30,000 psi system operating at 300 F? Hmmm... a bit strange

39Machine, do you mean to say that this vessel has a 4" blank and the torque is actually to be applied to the bolts in the blank and mating flange?

I was imagining a blank also -- but an incredibly large one held in place by 4" fasteners.

Pressure vessel 30000psi + 300 deg = READ THE JOLLY MANUAL before someone dies......please!

Damage the shoulder by over torquing and you have a sizeable projectile with what ever is in the vessel waiting to take a ride in who knows what direction the failure chooses. It's going to be a bigger mess than a gemsquash popping in your microwave!