Understanding Bolts
Importance of Using the Right Bolts
Getting to The Nuts and Bolts of It
Nuts and bolts might not be the first things that come to mind when we think about valve maintenance and repair, but they’re definitely worthy of their share of attention.
The nuts and bolts are what hold everything together. Fasteners serve one of two functions: either holding valve components in place or clamping two parts together.
Stretching
Bolts are meant to stretch a little. The stretching is what gives them their force or clamping ability.
As with anything that stretches, fasteners can be overstretched causing them to lose their elasticity or ability to stretch. Think of a spring that’s been stretched out too far. It doesn’t return to its original shape. The same is true of a bolt. A bolt has an elastic limit. It should be torqued to just under its elastic limit. It stretches a little as it’s tightened. If it’s stretched too far–to the point just before it breaks–it will never return to its original size and shape.
You may have guessed then, that a fastener that has been overstretched and can no longer do its job (clamp or hold things in place) needs to be replaced, otherwise failure may occur.
Another factor that may lead to the need for replacement of fasteners is that there could be corrosion, thus reducing their strength or elasticity.
When it’s time to replace them, any old bolt won’t do.
Choices in Fastener Material
Like the valves themselves, nuts and bolts come in many types of metals. Selection of bolting material is based on several aspects such as:
- base material of the valve
- pressure class of the valve
- operating temperature
- nature of the media
- placement or location of the fastener
- whether or not it will be in contact with the media Here are the major players:
Cast iron valves
Where cast iron valves are used, the pressure and temperature are typically mild to moderate. ASTM A307 grade A bolt material along with ASTM A563 for the nuts is usually sufficient.
Steel and alloy valves
Material with high strength, moderate tensile strength, and moderate elongation qualities are a good choice for steel and alloys. ASTM A193 for bolts and A194 for nuts are generally chosen.
Carbon steel valves
When at moderate temperatures, A193 grade B7 bolts, and A194 grade 2H nuts are common for carbon steel valves. For Cr/Mo valves, A193 grade B16 bolts and A194 grade 4 (or occasionally grade 2H) nuts are recommended. B16 bolts have a higher tensile strength and elongation minimum than those mentioned above.
Austenitic stainless steel valves
Stainless steel fasteners are usually used for austenitic steel valves. A193 grade B8 and B8M bolts and A194 grade 8 and 8M nuts are typical.
Risk of Environmental Stress Corrosion Cracking (SCC)
Before the final decision can be made on the best material for the nuts and bolts, there’s another variable to look at corrosion. In situations where the risk of corrosion is normal or moderate, the general guidelines don’t require modification. Where the valves are subject to harsher environments with exposure to corrosives, either internally or externally, the need for corrosion resistance will require adjustments to the standard materials.
Environmental stress corrosion cracking differs from general corrosion. General corrosion can be seen. SCC, on the other hand, can occur without warning and usually is not visible. Materials that protect against SCC may not stand up as well to general corrosion. Because general corrosion will be visible though, corroded nuts and bolts can be replaced well before failure occurs. For this reason, it is better to choose the material that will resist the invisible stress corrosion damage that can bring sudden valve failure.
SCC can be caused by chloride, hydrogen sulfide, caustic embrittlement, and liquid metal embrittlement. Let’s take a look at each.
Chloride stress corrosion cracking (CSCC)
Chlorides and other halogens can cause SCC of austenitic stainless steels such as 304 or 316. Where chloride is present alloy steel fasteners should be used. Marine environments are corrosive to these carbon steels, so coating the alloy steel with a metal like zinc or cadmium will reduce the susceptibility to general corrosion. These platings are useful for protecting carbon steel fasteners under moderate temperatures.
Liquid metal embrittlement (LME) and solid metal induced embrittlement (SMIE) Unfortunately, under high operating temperatures zinc or cadmium plating can lead to liquid metal embrittlement or solid metal induced embrittlement. LME is the brittle failure of a normally ductile metal when in contact with a thin film of liquid metal and stressed in tension. It can occur when zinc and cadmium, which have low melting points (489°F zinc and 370°F cadmium), are heated to near or above that point. When the temperature reaches the melting point, the thin layer of liquid metal over the ductile carbon steel becomes liquefied and can cause LME. SMIE is pretty much the same, occurring below the melting point. Avoid LME and SMIE by restricting the use of zinc or cadmium to lower temperatures.
Hydrogen sulfide stress cracking (HSSC)
Highly corrosive hydrogen sulfide causes stress cracking or brittle failure in most highly stressed hard steels. Softer alloy fasteners such as A193 grade B7M and A194 grade 2HM help to alleviate this. They’ve been heat-‐treated to reduce their hardness and strength making them less susceptible to HSSC. The details for fastener requirements in refineries are laid out in NACE International specification MR0103.
Installation
Proper installation of nuts and bolts goes beyond the selection of the right materials. There is a correct way to install fasteners. Always pay careful attention to the following factors:
- Tightening must be done using a crosswise pattern to ensure even compression of all surfaces forming a seal.
- Employment of the correct torque is essential. Always look for information on the bolting torque before installation. CPV Manufacturing provides the information for our product torque values here: torque valves.
- The current industry best practices are available in ASME’S PCC‐1 ‐ 2013, “Guidelines for Pressure Boundary Bolted Flange Joint Assembly,” which should be kept on hand wherever pressure-containing bolted joint assembly takes place.
Fastener selection isn’t always complex or challenging, but it can be. Be sure to give the nuts and bolts the attention they require. It could be the difference between reliability and catastrophe.
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