Valve Magazine - Fall 2009

MAINTENANCE & REPAIR

BY GREG JOHNSON

The Nuts and Bolts of Nuts and Bolts

Repairing a valve usually means discarding old fasteners and replacing them with new ones. But it takes more than just matching thread diameter and pitch to make the correct fastener selection.

First let’s discuss why a bolting change would be necessary. Nuts and bolts serve two purposes: they hold components in alignment or they provide clamping force. In some cases, they do both. In the case of valve body/bonnet joints, the fastener’s function is to clamp the two pressure-containing valve halves tightly together with a gasket between them to prevent fluid leakage. When a bolt is tightened, it is stretched a small amount. If it was not stretched, there would be no clamping force.

If a bolt has been stretched beyond its ability to yield while in service, however, the fastener may have lost its ability to stretch and clamp. Also, there may be corrosion damage to the bolt, which can reduce its overall strength or ability to stretch. Both of these situations are reasons to change bolting when a valve is repaired.

Bolting for valve applications comes in many materials that are chosen based upon the base materials of the valve, that valve’s pressure class and the service conditions in which it will be used. The correct combination of strength and stretch in a bolt makes it function well as a clamp.

In the case of cast iron valves, the pressure, temperature and service requirements are usually not severe, so a “standard” grade of bolting will suffice. In most cases, this will be ASTM A307 grade A material, which has a minimum tensile strength of 60 ksi

(thousands of pounds per square inch) and a minimum elongation of 18% (elongation is stretch measured before deformation). These bolts will have 307A marked on them. The companion nuts for these bolts are detailed in ASTM A563.

Moving up to steel and alloy valves, such as API gate and check valves or ASME B16.34-designed valves, the bolting requirements are usually through ASTM A193 (bolts) and A194 (nuts). These are high-strength materials with tensile strengths from 75 ksi to 125 ksi minimum. Their minimum elon-

gation requirements are from 16% to 35%, depending on grade.

For plain carbon steel valves, such as ASTM A216 grade WCB and forging grade A105, A193-grade B7 bolts and A194-grade 2H nuts are the most common. When high temperatures dictate the use of Cr/Mo valves such as ASTM A217 grades WC9, C5, C12 & C12A, B7 bolting is insufficient. In these cases, A193 grade B16 bolts and A194 grade 4 nuts should be used. Occasionally, grade 2H nuts are also used with B16 bolts. B16 bolts have a minimum tensile strength of 125 ksi and minimum elongation of 18%.

Austenitic stainless-steel valves (304 & 316ss) usually require stainless-steel bolting. The common grades of stainless-steel bolting used are A193 grade B8 (304ss) and B8M (316ss) with A194 grade 8 (304ss) and grade 8M (316ss).

For low temperature applications down to – 50° F (– 45° C) in valves such as A352 grades LCB & LCC and A350 grade LF2, ASTM A193 grade L7 bolts and ASTM A194 grade 7 nuts are commonly used.