Environmental agencies and media claim: Dilbit is a highly corrosive, acidic and potentially unstable blend of thick raw bitumen and volatile natural gas liquid condensate-raising risks of spills and damage to communities (in terms of any pipeline flowing along the pathway).
This is absolutely untrue. When bitumen is produced by surface mining or in-situ methods it is heavy and cannot be transported in pipeline (API 6-10) and upgraded, it needs processing prior to selling/transporting into pipeline/s.
The raw bitumen is mixed with light oil/diluents, also some is converted into Synthetic Crude Oil(SCO).
A few categories of OilSands products are produced and converted into gasoline, diesel, etc:
- Diluted bitumen (commonly known as DILBIT): When there is 70% bitumen and 30% diluents on a volume basis to make the mixed product lighter to meet pipeline specifications for density and viscosity to flow easily and be transported safely in pipeline/s. The diluent is typically a natural gas liquid also known as condensate or naphtha. The dilbit is heavier than light crude oil, with approx.. API22 grade.
- Synthetic Crude Oil (SCO, this can be sweet or sour): This is produced from bitumen by converting into a product which is lighter than dilbit and bitumen. This is produced by hydrotreating, and comes with approx.. API33 grade.
- Synthetic Bitumen: A combination of 50% bitumen and 50% synthetic crude oil with approx. API28 grade.
In this article we will discuss materials used in a valve body, trim and soft components utilized in such rigorous applications:
Non slam check flow. Image courtesy of Mokveld Valves BV.
Generally speaking body and trim materials used in the valve industry are the same, for example bronze valves have brass or bronze trim and cast iron valves have bronze trim, however carbon steel or alloy steel valves are available with a wide range of trim materials including bronze and stainless steel.
Manufacturers of cast steel gate, globe and check valves offer casting materials in low temperature carbon steel ASTM A352 Grade LCB/LCC and forgings in ASTM A350 Gr LF2. This steel has the same composition and applications as plain carbon steel, but it is heat-treated to produce similar physical properties at low temperatures. It is recommended for service down to -50° F to +650° F.
ASTM A217 GR WC6 (castings) and ASTM A182 Grade F11 (Forgings) contain 1.25% chromium – 0.5% molybdenum alloy steel. The addition of chromium and molybdenum to carbon steel improves high temperature strength and resistance to graphitization and creep. This composition is recommended for valves operating at temperatures up to 1000° F.
NiAIBr tricentric butterfl y valve. Image courtesy of Score Valves Canada.
ASMT A217 Grade WC9 (Castings) and ASTM AGr F22 (Forgings) contain 2.25% chromium and 1% molybdenum alloy steel. This alloy has higher resistance to creep and graphitization than Gr WC6 castings. Similarly this grade is recommended up to 1000° F. These grades require pre-heating and heat treatment after welding.
ASTM A217 Grade C5/ASTM A Grade F5 contains 5% chromium alloy steel and ASTM A217 Grade C12/ASTM A182 Gr F9 contains 9% chromium alloy steel. Such castings have higher resistance to corrosion, erosion and scaling. It is recommended for oilsands, refining with temperatures up to + 1050° F.
ASTM A351 Grade CF8 in castings and ASTM A182 F304 in forgings i.e. 18-8 austenitic stainless steel. This 18% chromium – 8% nickel stainless steel is suitable for oxidizing and very corrosive fluids. It is recommended for extremely corrosive oil and liquid oxygen service and is also suitable for nitric acid from -150° F to +1000° F. This material is easily weldable without any heat treatment.
Non slam control flow. Image courtesy of Mokveld Valves BV.
ASTM A351 Grade CF8M in castings and ASTM A182 F316 in forgings i.e. 18-8-3M austenitic stainless steel. The addition of 3% molybdenum to 18-8 stainless steel results in improved resistance to corrosion and high and low temperatures for oil, diluted hydrochloric acid, brine and other process fluids. This material is easily weldable.
ASTM A351 Grade CN-7M in castings – also known as Alloy 20. This contains 20% chromium and 29% nickel alloy and possesses excellent resistance to sulfuric acid with good concentration and this is extensively used in the processing of brine, produced water, and refining.
Manufacturers refer to combinations of metal trim materials as per API600 Trim Table.
The bolting materials should be compatible with the other body material and in compliance with valve standards. The commonly used bolting materials for example are:
ASTM A193 Gr B7: Alloy and stainless steel bolting materials for high temperature service up to 1000° F is a standard material in steel valves.
ASTM A194 Grade 2H : Carbon and alloy steel nuts for high pressure and temperature service, is a standard grade nut for use with B7 studs and bolts.
The most common gaskets used in valve manufacturing are flat, spiral wound and ring joint gaskets. Flat gaskets are used on low-pressure valves and are made of both metallic and non-metallic.
Spiral-wound gaskets are made of alternate piles of preformed metal bands and non-metallic filler.
Ring-joints are metal.
The basic packing material in valve stem and shaft packing is braided carbon fiber and synthetic yarns. Commonly Inconel wire is twisted with, or inserted into, the yarn for added strength. Flake graphite or molybdenum disulfide are used as lubricants and sacrificial metals such as zinc are added to inhibit corrosion.
Molded plastic: formed rings of solid plastic, typically TFE, are used in low temperature, high corrosion applications.
Die-formed: flexible graphite foil is slit into ribbons and corrugated. The ribbons are then wound in dies and compressed into dense rings, along with a corrosion inhibiting agent.
When used with braided top and bottom rings that act as wipers and limit extrusion, these rings are very effective packing sets for use against gas and vapor leaks.
While designing quarter turn ball valves, a few applications require special attention to the trim requirements such as:
If solid particles are contained in the fluid, valves are provided with Tungsten Carbide (WC) coated metal ball and seats to avoid erosion of the sealing surfaces.
High Temperature Service
Some processes require valves able to be operated and to assure leakage rates within specified limits at high temperatures. Valve manufacturers provide a special valve design including extended bonnet (recommended for use at temperatures over 250°C), gaskets and seals and material selection suitable for high temperature service, fin tubes to dissipate heat and for application where soft seats would be unsuitabl. Tungsten carbide (WC) or chromium carbide (CrC) coated metal ball and seats are used.
High Pressure Service
Trunnion ball valves can be properly designed for very high pressure ratings such as per API6A 2000 to 20000 and ASME B16.34 4500 classes. Manufacturers perform special tests as per API 6A requirements.
Some gas treatment processes require valves able to be operated and to assure leakage rates within specified limits at low temperatures. Valve manufacturers provide a special valve design including extended bonnet (recommended for use at temperatures below minus 50°C), gaskets and seals and material selection suitable for cryogenic service.
Sour Service(API6D Pipelin Valves)
Packing, exploded view. Image courtesy of
Materials for pressure-containing parts of valves used in H2S-containing environments in oil and gas production shall meet the requirements of NACE MR0175 and ISO 15156 (NACE). HIC and SSCC corrosion test certificates.
High alloys, nickel alloys, duplex (22Cr – 2 Ni) and superduplex (25 Cr – 5 Ni) can be supplied according to corrosion resistance requirements as per special material standards (i.e. Norsok MDS D46 and D44 for duplex, D56 and D54 for superduplex, or similar standards). In addition Electroless Nickel Plating (ENP) and 316L SS or Alloy 625 weld overlay are frequently used to enhance corrosion resistance of balls, seats, stems, seat pockets and stem sealing areas, in corrosive service.
Zero Emission Valves
Ball valves are often required as “Zero emission valves” that is, able to comply with the updated regulations which state very low rates of emission from the valve to the atmosphere. In this case special valve design including high body and stem surface finishing and suitable seals is available.
Anti explosive decompression O-rings or lip seals are used to eliminate the possibility of O-ring explosion due to the sudden decompression of gas absorbed by the molecular structure of elastomeric sealing elements.
Erosive and corrosive fluids such as carbon dioxide, chlorine, oxygen
Valves working with particular fluids, must meet some special requirements according to valve manufacturers’ special procedures. To avoid risk of explosive decompression in carbon dioxide (CO2) service special O-rings or lip-seals are used. Valves to be used with chlorine need special features and a full degreasing and decontamination. Valves for oxygen service also require special features to eliminate the risk of ignition.
To sum up, the valve manufacturing industry acknowledges the challenges of process and production and comes up with a solution tailored to applications meeting current and updated industry regulations, codes and compliances.