Latest Chemical Interview Questions Part – 7

What Are Some Of The Consequences Of An Undersized Kettle Type Reboiler?

The effect will be a decrease in the boiling coefficient. A boiling coefficient depends on a nucleate boiling component and a two-phase component that depends on the recirculation rate. An undersized kettle will not have enough space at the sides of the bundle for good recirculation.

Another effect is high entrainment or even a two-phase mixture going back to the tower. Source: Gulley Computer Associates.

What Are Some Good Strategies For Curing Tube Vibration In Shell And Tube Exchangers?

Most flow-induced vibration occurs with the tubes that pass through the baffle window of the inlet zone. The unsupported lengths in the end zones are normally longer than, those in the rest of the bundle. For 3/4 inch tubes, the unsupported length can be 4 to 5 feet.

The cure for removable bundles, where the vibration is not severe, is to stiffen the bundle. This can be done by inserting metal slats or rods between the tubes. Normally this only needs to be done with the first few tube rows. Another solution is to add a shell nozzle opposite the inlet to cut the inlet fluid velocity in half.

For non-removable bundles, this is the best solution. Adding a distributor belt on the shell would be a very good solution if it were not so expensive. Source: Gulley Computer Associates

Is There A Quick Rule-of-thumb To Estimate A Gas Side Heat-transfer Rate Inside The Tubes Of A Shell And Tube Heat Exchanger?

If you need to estimate a gas heat transfer rate or see if a program is getting a reasonable gas rate, use the following: h = 75 X Sq. Root(Op. pressure/100) The operating pressure is expressed as absolute. This is for inside the tubes. The rate will be lower for the shell side or if there is more than one exchanger. Source: Gulley Computer Associates

What Factors Go Into Designing The Vapor Space Of Kettle Type Reboiler?

The size of the kettle is determined by several factors. One factor is to provide enough space to slow the vapor velocity down enough for nearly all the liquid droplets to fall back down by gravity to the boiling surface. The amount of entrainment separation to design for depends on the nature of the vapor destination.

A distillation tower with a large disengaging space, low tower efficiency, and high reflux rate does not require as much kettle vapor space as normal. Normally the vapor outlet is centered over the bundle. Then the vapor comes from two different directions as it approaches the outlet nozzle.

Only in rare cases are these two vapor streams equal in quantity. A simplification that has been extensively used is to assume the highest vapor flow is 60% of the total.

In one case, where this would cause an undersized vapor space is when there is a much larger temperature difference at one end of the kettle then the other. The minimum height of the vapor space is typically 8 inches. It is higher for high heat flux kettles. Source: Gulley Computer Associates

What Kind Of Concerns Is Associated With Temperature Pinch Points In Condensers?

Be extra careful when condensers are designed with a small pinch point. A pinch point is the smallest temperature difference on a temperature vs heat content plot that shows both streams. If the actual pressure is less than the process design operating pressure, there can be a significant loss of heat transfer.

This is especially true of fluids that have a relative flat vapor pressure plot like ammonia or propane. For example: If an ammonia condenser is designed for 247 PSIA operating pressure and the actual pressure is 5 PSI less and the pinch point is 8 0F, there can be a 16% drop in heat transfer. Source: Gulley Computer Associates

What Are Some Common Causes Of Gas Pipeline Vibration 20 Carbon Steel Line?

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.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.

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.

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.

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:

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

What Is A Good Way To Get Started In Doing A Plant-wide Steam Consumption Analysis?

It is unclear as to whether or not you know the total steam consumption. If you do not, one way to get it is to take the nominal capacity of the boiler in terms of heat, i.e. the total rated Btu/hr. This is usually available either through the documentation you have for the boiler or even on the nameplate.

You also must know the steam pressure you are producing. Using the steam tables, get the enthalpy of the steam and divide it into the nominal boiler capacity to get the total rate. I hope that you also know how much of the capacity you are using, 50%, 75% etc. Multiply this by the total lb/hr to get your rate.

Another way to get the capacity is by using the amount of boiler feed water you are sending to the boiler and the known level of steam you are producing. Do not forget to include the blow down in your heat & mass balance. Getting the rate to each plant is more difficult if you are lacking in instrumentation.

Use as much plant instrumentation as possible; flow meters, pressure and temperature indicators. If you do not have a meter in each header to each plant, then see if you have them in sections or to pieces of equipment using the steam. Another way is to measure the amount of condensate you are returning to the boiler.

If you are dumping the condensate, you may be able to collect and measure the amount in a pail from each source. Another way is to use the process instrumentation and do some mass and energy balances around the steam users.

Are There Any General Rules For Flushing Slurry Lines?

Slurry lines should be flushed with a minimum fluid velocity of 10 ft/s and the total flushing liquid volume should equal 3-6 times the total piping volume.

How Can You Determine The Proper Pipe Thickness For A Slurry Line?

Design of slurry piping systems should follow ANSI/ASME B31.1 and B31.11 Codes. A simple equation for this calculation is as follows: t = (PD) / (2S) + C where: t = pipe wall thickness, in. P = maximum design pressure of the pipe, psig S = maximum allowable design stress, psig C = corrosion or erosion allowance, in.

What Is The Best Way To Handle Bend Or Turns In Slurry Piping Systems?

Even long radius elbows should be avoided in slurry pipes and lines. They are often the site of severe erosion or solid/liquid separation. Only gentle pipe bends or sweeps should be used to turn a slurry line. Industrial experience has shown that a bend-radius-to-pipe-diameter ratio of 3-5 is recommended.

How Can You Prevent Bridging In A Dilute Phase Pneumatic Conveying System?

Manufacturers of these systems recommend bin agitation or blowing air into the top of the feeding bin. These methods can prevent fine particle from bridging near the rotators valve. Two types of particles that are especially prone to bridging include titanium dioxide and calcined- kaolin clay.

What Is Some Common Piping Materials Used To Transport Slurries?

When selecting a piping material to transport slurries, corrosion and erosion considerations must be
accounted for.

Some of the most popular piping materials include:

Carbon Steel
Stainless Steel
High Density Polyethylene (HDPE)
Acrylonitrile butadiene styrene (ABS)
Unplasticized polyvinyl chloride (uPVC)
Fiberglass reinforced plastic (FRP)
Elastomer-lined carbon steel