Chemical Engineering Interview Questions and Answers

1 :: What are some common causes of gas pipeline vibration 20 Carbon Steel line?

Check / consider the following:

► The upper pressure range and /or the smaller pipe diameters prompts me to investigate the possibility that the gas is reaching critical flow somewhere downstream within the pipe. When a gas gets to critical flow, sonic booms (producing vibration) are expected. In fact, one of the main means by which the additional pressure in the pipe is lost.

► If the source is a compressor, look for surging.

► If the source is a tower, look for pressure cycling in the tower

► Look at critical flow through any control valve that may be in the line.

► Are there any vapors in the line, which can condense and produce two-phase flow? Two-phase flow can cause vibration.

In chemical plant design, if we suspect two-phase flow, we instruct the piping designers to provide special anchoring.

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2 :: While there, are many tests available to detect leaks on vessels, is there a technology available to quantify the leak, or measure the flow through a leak?

The RheoVac air in-leak monitor by Intek, Inc. in Westerville, OH is a viable meter that gives the actual air in-leakage flow rate. It also gives you exhauster capacity and a vacuum quality reading. If you want to find more information, you can view their web site below.

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3 :: Is there any way to repair a valve that is passing leaking internally without taking our process offline?

A 600 psig, 3" steam line is experiencing "passing" or internal leakage. If you order to replace the valve, the process would have to be taken offline. A temporary solution to the problem is sought to get the plant to their next scheduled shut down ANSWER Research on-stream leak sealing services. This problem is quite common. What they would do in this case is drill a hole into the bypass valve on the upstream side but not completely into the line. They would then tap the hole and install one of your injection fittings, which is like a small plug valve. They would then take a long 1/8" drill bit and drill through the open injection fitting and into the pressurized line. The drill bit is then removed and our injection equipment is then attached. Sealant (heavy fibers and grease) is pumped into the line and caught in the flow, which will bind up against the leaking seat on the bypass valve. If done properly, this technique can be both effective and safe.

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4 :: When using a pumping loop to mix two miscible fluids in a tank, when can the content are considered well mixed?

A rule of thumb is to turn the tank over three (3) times and then sample the tank for mixture properties. By "turn the tank over”, we mean to force the entire volume of the tank through the pump at least three times. More turnovers may be necessary, but three times is a good starting point.

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5 :: What are some good uses of low-grade steam at 12 atm and 1920C?

There are various traditional methods to employ waste steam in an operating plant:

1. You can generate electricity through a steam turbine-generator set. The electricity is usually put back in the line; this is the idea behind the "Co-Gen" concept used today in many USA plants. Steam turbines can effectively use saturated steam supply down to 75 - 100 psig. In special conditions, they have used down to 50 psig as a turbine steam supply. I have used steam as low as 100 psig.

2. You can pre-heat process streams that require pre-heating; this is done by applying heat exchangers.

3. You can employ the waste steam as a refrigeration source by employing it in vacuum jet ejectors and producing 50 of cooling water.

You have to consider these as viable options if you can identify the heating, cooling and energy conservation requirements. An economic analysis is required to identify the most attractive option. You usually utilize a Discounted Cash Flow analysis to base your decision and that means you must study each case as to savings generated. A fourth method might be that you can use the steam for environmental heating (if you live in a cold climate).

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