Dual Certified Vessels for Low Temperature Service
Many existing carbon steel vessels built before modern code rules do not meet current code requirements for low temperature service. The service history on these vessels is good resulting in an attitude that the current code rules are too restrictive. However, there is an alternate way to understand these vessels: many are in service conditions like propane storage where the pressure at lower temperatures can never get very high. Many of these vessels can never experience situations of combined high pressure and low temperature.
Propane Pressure Temperature Curve:
A propane storage vessel typically has a pressure rating of 250 psi. In Canada it can also have -50°F MDMT (Minimum Design Metal Temperature) depending on the service location. What it does not experience is 250psi at -50°F. The pressure temperature curve for Propane is shown below. Some data points have been highlighted on the propane P-T curve. At -50°F, a common design temperature for exposed locations in Canada, the pressure in a propane storage vessel is a small vacuum of -2 psig. At -20°F, a common minimum temperature for older design codes, the pressure is only 11 psi. At 250 psi, a common propane design pressure, the temperature is 127°F. Once the contents of the vessel reach this temperature, a 250 psig relief valve will open allowing the release of some gas reducing the temperature of the remaining contents through boiling.
Data Source: VirtualMark.net
A Typical Vessel Design:
A sample vessel was designed with a minimum wall thickness to just pass 250psi service. The material category was changed from Curve A to B to C to D. As the pressure was reduced the minimum allowed temperature reduced depending on the material curve used. A crude generalization is that Curve A represents coarse grained materials with poor low temperature impact (toughness) properties. Curve D materials have the best properties obtained through methods like normalization or quench and tempering. Curve B and C are intermediate materials. This applies only to carbon and low alloy steels. The list of materials and their curve is found in Fig UCS-66. Through application of ASME rules built into all commercial code calculators, the curves below were generated:
In this case, all material curve variants for this sample vessel have pressure ratings above the propane PT curve, so all could be used safely, but Curve A in this case could not reach -50°F. The Curve A material can be seen to have the poorest pressure rating at low temperature, and the Curve D the best. When a vessel is designed for new construction it is possible to combine the selection of material with the appropriate testing to obtain MDMT of -50°F, even at full pressure. However, for this sample vessel, no material combination provides a full pressure rating at -50°F.
Continuing this sample assuming Curve B material: ASME has additional rules like UG-20(f) that allow higher pressures to be used for some materials down to -20°F service. With UG-20(f) applied, the Material pressure temperature curve for a Curve-B vessel now looks like:
The design has been dual rated: 250 psig at down to -20°F and 115 psig for down to -50°F. This is recorded according to the rules of UG-116(a(5) footnote 37 which does not restrict the number of minimum temperature pressure combinations used. One full calculation set is required for each P-T combination. Two full calculation sets are required for this vessel: the first is calculated at a pressure of 250 psig and shows a minimum temperature of -20°F; The second is calculated at 115 psig and shows a minimum temperature of -20°F.
As an alternative to using these curves, it is possible to impact test materials and welds. See the comment from ABSA at the end of this article. In general it does not pay to be optimistic about coarse grained materials and welds passing impact tests prior to seeing the actual test results.
More on SA-212 material:
ASTM SA-212-39 (S-55) was put into Section II in the 1940 edition of the Code. There were two grades in S-55: A and B, each with two different minimum tensile strength requirements controlled by carbon content.
In 1952… it was required in SA-212 that plates intended for low-temperature service must meet the impact requirements in SA-300. SA-212 could be purchased to a fine-grain-melting practice-and subsequently normalized and tempered-for low temperature service, or purchased to a coarse grain-melting practice; the single specification permitted the manufacture of both plate grades. The SA-212 Specification continued up to 1962 as the carbon steel plate material of choice for low-temperature service for boiler drums and pressure vessels.
[In the] 1968 edition of Section II … the SA-212 Specification was deleted …it was replaced with two specifications. The SA-212 steel plate melted to coarse-grain practice was replaced with SA-515 (Specification for Pressure Vessel Plates, Carbon Steel, for Intermediate and Higher Temperature Service) and the SA-212 steel plate melted to finegrain practice was replaced with SA-516 (Specification for Pressure Vessel Plates, Carbon Steel, for Moderate and Lower Temperature Service). These two specifications-along with SA-299 (Specification for Pressure Vessel Plates, Carbon Steel, Manganese-Silicon), which has slightly higher room-temperature strength-were first published in the 1949 Edition of Section II. They continue to be used today as the carbon steel plate materials of choice for boiler and pressure-vessel applications.
SA-212 in older vessels being recalculated could be either coarse or fine grain. Either Curve A or a better curve. Proving that it was built to fine grain practice and impact tested to SA-300 could be difficult for an old vessel. Sometimes it is only possible to assume that it was made to Curve A. There are also some concerns that special care is required for hydrotesting coarse grained pressure vessels. See TSSA.
When a used vessel moves to a new location in Canada a new CRN registration number is usually required. The CRN calculations are based on the inspected wall thickness. Three possible calculation methods are used: 1) calculations to be based on current code rules (see note from ABSA below); 2) Calculations based on code rules at time of construction or; 3) Calculations based on both the time of construction and the current code rules, the most conservative to be applied. Typically a calculation set to one of the above methods is prepared to create a submission package and get a review engineer assigned. At that point the calculation method can be changed to the assigned reviewer’s requirements. Also note that some Canadian reviewers/jurisdictions do not allow vessels to be dual rated. Some of our customers place reserve bids on used vessels and do not complete the transactions until the CRN has been obtained. It is important to get this sorted out before moving the vessel! More info from ABSA:
1.Q2. Is it permissible to bring into and operate a used pressure vessel that was manufactured of SA-212 Grade B steel? The vessel was not impact tested when it was manufactured.
1.R2. A used pressure vessel made of SA-212 Grade B steel may be brought into and registered for operation in provided that its proposed design conditions meet the intent of the current ASME Pressure Vessel Code. Since the current Code requires a minimum design metal temperature (MDMT) for a pressure vessel, such an MDMT must be established for the used vessel using the current Code methodology. SA-212-B material would be considered a Curve A material for the purposes of Code paragraph UCS-66. Therefore, a maximum allowable working pressure (MAWP) that supports the MDMT without impact testing would have to be established. It is assumed that it is not feasible to impact test all the shell and head plates and weld joints to support an MDMT lower than that without impact testing.
Laurence Brundrett President, Pressure Vessel Engineering Ltd. Aug. 16, 2012 PVE File 5920