Designing and Manufacturing Equipment For Low Temperatures
This is a follow up post to the Effects of Low Temperature on Performance of Steel and Equipment.
The initial article covered the background theory on low temperature steel and now I would like to review a bit more about how it affects design and manufacturing.
Many common engineering calculations are based around an assumed hardness of a material. Temperature can have a huge effect on the hardness of materials when approaching the ductile to brittle transition temperature. The problem is the transition temperatures of a few common structural steels are right around the temperature of a cold Canadian winter (or even spring). Engineers and end users need to be aware of this. It is really common for engineers to specify a minimum operating temperature of -20 degrees Celsius because it is easy and cost effective, knowing full well that the equipment could potentially see temperatures lower than that during its life cycle. In cases where this isn’t acceptable, (such as when the customer explicitly requests a minimum operating temperature, or if the product is going to be subjected to impact loading) switching to a quenched and tempered steel with a working temperature down to -40 degrees is a suitable solution. The problem is the grey area in between where a product has a -20 degree Celsius rating and is being used at -30 degrees Celsius. There are a few different codes that touch on low temperatures and steel but not all give clear direction on handle this situation.
How Material and Equipment Codes Address Low Temperatures
- API 7K – A code about drilling and well servicing equipment including drawworks design among other things and is very specific regarding low temperature testing. It specifies that primary load-carrying components with a specified minimum operating temperature require three full sized test pieces tested to ASTMA370. These test pieces need an average V-notch Charpy result of at least 20 ft-lbs at or below the specified operating temperature.
- ASTM A370-12a – The code published as a guide to the physical testing of materials. Most companies won’t ever need to worry about this test because these procedures either happen at a metallurgic lab or at the mill. However, this is a code that gets referenced a lot. ASTM A370 goes into specific detail about calibrating test procedures, preparation of test samples, and how to properly interpret the data. In past projects, it has been important for Sparta to be aware of the test procedure so that enough material gets ordered to properly prepare the test sample as well as complete the part. We have not had to perform an actual material test yet.
- B30.20 – The code that governs below the hook lifting devices states that additional considerations need to be taken if the working temperature is outside the range of -4 degrees C to 66 degrees C. It suggests that engineers either de-rate the capacity or use steel that is better suited for low temperature service. It goes into further detail as what to do in industrial applications where the below the hook lifting device is subjected to repeated high temperature heat cycles. National Crane has followed this guideline and de-rates their cranes 3.67% for every degree below -40 degrees C that the crane is operated.
- B30.26 – Which governs lifting slings and shackles recommends t users seek the manufacturer or qualified person if operating temperature is above 204 degrees C or below -40 degrees C.
Sparta Engineering’s Experience with Design for Low Temperature
Case Study #1 – Customer Specifies Operating Temperature of -40 degrees C
Sparta Engineering has an affinity for using quenched and tempered steel in structural applications. Low temperature operation is less of a problem then a company using primarily mild steel, however, we have had one notable experience with low temperature on a 7k drawworks. In this case the customer requested a API 7k drawworks with an operating temperature of -40 degrees C. In the engineering world this was fairly straightforward. Following API 7k guidelines we specified materials with a minimum charpy hardness test at -40 degrees C and manufacturing procedures that matched.
On the fabrication side of this project, this had substantial repercussions. There were two main obstacles we had to overcome that would determine the success of the project.
Hurdle #1 – Material Traceability: The first hurtle was part of material traceability. API 7k requires all material to be traceable and match specifications. This means that the MTRs needed to explicitly state the hardness at the customer specified temperature of -40 degrees. This was difficult to achieve since the hardness to temperature relationship is not linear. You can’t extrapolate test results done at different temperatures. So we asked the suppliers to find material that has an actual test performance that met our criteria but it didn’t seem to exist for some material. Ultimately, we decided to commission a third party lab to conduct material testing but it was also interesting to find that the steel suppliers didn’t seem particularly equipped to deal with this kind of request. We were using some special high tensile material and needed a sample to make into coupons and send to the lab. The steel suppliers were reluctant to give or sell us a sample and were instead expecting us to purchase the entire length of material prior to knowing whether it met our hardness criteria or not.
Hurdle #2 – Manufacturing Procedures: The second major hurtle to overcome with a low temperature specification was the manufacturing procedures. We were utilizing a few welds that were not pre-qualified to CSA W59 because of the material types. In cases where you are welding to CSA W59 standard but not using one of their pre-qualified weld procedures you are required to perform a procedure qualification record (PQR). This PQR was done a few years prior but didn’t include any hardness data because at the time it wasn’t required. New PQR’s needed to be made which requires welding out test coupons, conducting a bend test, and then sending away to a lab in order to get hardness testing done to the weld and the heat effective zone.
What to Consider When Working with Equipment in Low Temperatures?
Working with steel equipment in low temperatures should be taken seriously and safety precautions should be considered. As you can see from our example above, customers approach this problem in different ways. Some specify a low temperature rating for their equipment but not really understand the implications of doing so. Other customers don’t specify a minimum operating temperature and by default get -20 degrees Celsius. Equipment owners and operators need to be aware of the dangers of operating the equipment in cold temperatures. My intent with this article is to share some of my reflections on the subject and to make sure people that are in these kinds of situations understand what is happening.
Specifying a low operating temperature is not an insurmountable problem, however, it can have a substantial affect on the cost and delivery date of a project. Make sure you are asking the right questions.