Welcome to Hardhatengineer.com. I am Varun Patel. In this video, you will learn about the most common questions regarding piping that you may face during the interview.
- What is insulation gasket kit and where do you use it?
- What is insulation joint?
- What is the minimum distance to be maintained between two welds in a pipe?
- What are different types of insulation used in piping?
- What is the difference between GRE and GRP Pipe?
- What is the difference between post heating of weld and PWHT?
So please watch this video till the end. And don’t forget to subscribe to my channel because that way you will get regular updates on a new video. You can subscribe right now by just clicking subscribe button on your screen. So let’s start with the first question.
What is insulation gasket kit and where do you use it?
The insulation gasket kit is a specially designed gasket package that use to prevent galvanic corrosion of flange face in a piping system when the material of connecting flanges is different. Here, in the image, you can see the complete kit of insulation gasket. Basically, it prevents direct metal to metal contact of two different flange material.
What is insulation joint?
Insulation joint is used to break the current flow of an underground pipeline that is equipped with cathodic protection. To protect the underground pipeline from corrosion, cathodic protection is used. An external current is supplied to the pipeline. However, if this current flow continues when the line goes above-ground, it will damage the paint that applied to protect the above-ground pipeline. To avoid this situation, insulation joint is used. It is designed in such a way that it will break the flow of current. Here in the image, you can see the configuration of insulation joint.
What is the minimum distance to be maintained between two welds in a pipe?
No, American code and standard mandate the minimum distance between two welds in a pipeline. But most company specification will give some minimum distance that you have to maintain between two nearby welds. This is to avoid overlapping of heat affected zones.
The thumb rule is that the minimum distance between adjacent butt welds is 1D. If not, it is never closer than 1-1/2″. This is supposed to prevent the overlap of heat affected zones. In the image, you can see what I am trying to explain to you. Here, A and B are two weld joints. The minimum distance between these two joints should be the one-time diameter of the pipe. So, if your pipe is 4” NPS the distance should be 4”. If this is not possible, it should not be less than 1½”.
The minimum spacing of circumferential welds between centerlines shall not be less than four times the pipe wall thickness or 25 mm whichever is greater. Here, you can see the example. If you have branch welding on a pipe, then the distance between two adjacent branches or nearest girth weld should be four times the pipe wall thickness or 25 mm whichever is greater. Please note that these are thumb rule and vary from company to company.
What are different types of insulation used in piping?
You know that two types of insulations are used in the plant for the conservation of heat. First is cold insulation with a cryogenic system that prevents heating of the system and hot insulation that used to protect heat loss to maintain the temperature of the system.
Commonly used hot insulation materials are:
- Mineral wool
- Glass wool
- Calcium Silicate
- Ceramic Fibre
- Foam Glass
Commonly used Cold insulation materials are:
- Expanded Polystyrene (EPS) Thermocole
- Polyurethane Foam (PUF)
- Polyisocyanurate Foam (PIR)
- Foam Glass
What is the difference between GRE and GRP Pipe?
- GRP that is Glassfibre reinforced plastic. GRE that is Glassfibre reinforced epoxy.
- GRE/GRP pipes are used in seawater and sewage transport applications. These pipes offer excellent corrosion resistance property over all other types of pipes.
- The main difference between GRE and GRP is the resin used for bonding the glass fiber. GRP pipe used Isophthalic Resin whereas GRE pipe used Epoxy Resin.
- GRE pipes are used for the industrial application also, as it can withstand higher temperature compared to GRP pipe. GRE pipes are used in the offshore platform and water treatment plant to handle salty water.
What is the difference between post heating of weld and PWHT?
If you know what post heating and PWHT is, you can easily answer this question.
In post heating, you raised the temperature of the weld joint to a predetermined temperature range (usually 250° C to 350° C) for a minimum period (usually 3 hours) before the weld cools down to room temperature. Post heating performed immediately before weld get cold when welding is completed or terminated any time in between. Post-heating is also known as a Dehydrogenation as it removes entrapped hydrogen from the weld. Post-heating is done to avoid delayed cracking in the weld.
PWHT that is known as a post weld heat treatment is performed on the completed weld to relieve the stress from the weld joint. This will soften the heat affected zone and reduce the hardness of the weld.
Now you can answer the difference between post heating and PWHT. Post-heating is used to avoid delayed cracking, and PWHT is used to relieve the stress from the joint.
This is the end of the video and also the end of the Piping Interview Question series. I hope you have enjoyed the videos in the series. Please do share and like these videos with your friends. Don’t forget to visit my website, hardhatengineer.com to learn more about piping. See you soon — goodbye take care.