- Published on Wednesday, 14 October 2009 14:45
- Written by by Greg Johnson
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 elongation 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.
Common Valve Materials and Bolting Selections
A193 Bolting grade
A 194 Nut Grade
WCB, WCC, WC6*, A105
LCC, LCB, LF2
WC9, C5, C12, C12A
NACE MR0103, WCB & A105
A307, Grade A
A563, Grade A
*Grade B16 bolts & grade 4 nuts are sometimes used for this grade.
IN SPECIAL CASES
Special consideration must be given to bolting selection when the media or environment contains hydrogen sulfide (H2S). This highly corrosive compound will cause brittle failure in many very hard and strong steels, such as alloy bolting, so softer (and slightly less strong) bolting is selected. For carbon steel valves, the usual choice is A193 grade B7M bolting and A194 grade 2HM nuts. These fasteners have been specially heat-treated to lower their hardness and strength, which makes them less susceptible to failure in H2S environments. Complete details on H2S compatible bolting for valves in refinery service can be found in NACE International specification MR0103.
For higher alloy valves, there are a number of special high alloy bolting choices in materials such as Inconel, Hastelloy and Monel. For non-pressure-containing applications such as the bolting of actuator adapter plates and valve extensions, A307 materials should be used as a minimum—not off-the-shelf hardware store items.
For the correct bolting to work in a valve application, particularly a body/bonnet joint assembly, the bolts must be installed correctly. Two important issues in correct bolting installation are: 1) tightening the bolts using a crossing pattern of bolt tightening, and 2) applying the correct torque on the bolts. In many cases the manufacturer of the valve will have published information on bolting torques. If that information is not available, a good resource published by ASME can help. This is document PCC-1-2000, “Guidelines for Pressure Boundary Bolted Flange Joint Assembly.” This document gives guidance on bolt torques and assembly sequence. Anyone involved in pressure-containing bolted joint assembly should have PCC-1 in the library.
If the fastener assembly being replaced contains Belleville washers acting as a live load, these washers should be carefully examined for damage and replaced as necessary. Belleville washers come in different materials, and if corrosion is an issue, a corrosion-resistant alloy should be chosen. If upgraded washers are not available, thick grease can be applied to the washer/bolt assembly to retard corrosion.
In valve repair, the fasteners are as important as any other component, and care should be taken to replace these bolts or upgrade them as necessary. Fasteners may be one of the less expensive parts of the valve, but poor fastener choices can create a weak link in the valve repair process; one with potentially catastrophic consequences. VM