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How to Determine Design Pressure

As process engineer dealing with process equipment, we need to determine design pressure. For a vessel, design pressure is called maximum allowable working pressure (MAWP). The MAWP determines the setting pressure of relief valve and must be higher than vessel’s operating pressure. Operating pressure is fixed by process condition.

Table below recommends a minimum differential between operating pressure and MAWP so that the difference between operating pressure and relief valve set pressure provides a sufficient cushion. If the operating pressure is too close to the relief valve setting pressure, small surges in operating pressure could cause the relief valve to activate too soon.

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How To Estimate Vessel Weight

Estimating vessel weight is not usual daily task of a process engineer. However, I found it useful to know how to estimate vessel weight, when we need to estimate vessel cost. In other chance, civil engineer also asks about vessel weight to design the equipment foundation.

To better estimate vessel weight, we need to understand vessel weight components. Those are weight of the shell, the weight of the heads, and the weight of internals, nozzles, pedestals, and skirts. Read More

Sweep or Purge Gas Requirement for Flare

In many gas plant, flares are always installed to burn waste gases containing combustible components, such as methane, carbon monoxide, and hydrogen. The waste gases are piped to a remote, usually elevated location, and burned in open flame in ambient air using a specially designed burner tip, auxiliary fuel, and in some cases, assist gases like steam or air to promote mixing for nearly complete.

As mentioned in several posts, combustion requires three ingredients: fuel, an oxidizing agent (typically oxygen in air), and heat (or ignition source).

Flare Purge Gas and Flare Sweep Gas

In flare system, a pipe connection for purging or sweeping purpose is usually installed. Flare purge gas means gas introduced between a flare header’s water seal and the flare tip to prevent oxygen infiltration (backflow) into the flare tip. For a flare with no water seal, the function of flare purge gas is performed by flare sweep gas.

Flare sweep gas means the gas intentionally introduced into the flare header system to maintain constant flow of gas through the flare header to prevent oxygen build up in the flare header. Read More

Sizing of Amine System for Natural Gas Sweetening

In this post I want to share how to size amine system for natural gas sweetening, especially for MEA and DEA. For other amine systems, the licensee should be contacted for detailed design information. In the end of this post, I share simple spreadsheet on how to estimate amine circulation rate, minimum contactor diameter, and reboiler duty of amine system.

Amine absorber

Amine absorbers provide intimate mixing between amine solution and sour gas. Typically, small diameter towers use stainless steel packing, while larger towers use stainless steel trays.

For preliminary design, a tray spacing of 24 in and a minimum diameter of capable of separating 150 to 200 micron droplets can be assumed. The size of packed towers must be obtained from vendor.

Commonly, amine absorbers are equipped with integral gas scrubber section in the bottom of the tower. The function of the scrubber is to remove entrained water and hydrocarbon liquids from the gas, therefore it protects amine solution from contamination.

Separate scrubber vessel can also be provided so that the tower height can be decreased. The vessel should be design using two-phase separators.

For MEA systems with large gas flow rate, a scrubber should be considered for the outlet sweet gas. It is because it will be helpful to reduce MEA losses in the overhead sweet gas. DEA systems do not require scrubber because vapor pressure of DEA is very low. Read More

Sizing Iron Sponge Units for Sulfur Removal in Natural Gas

In last post we learned about one type of sulfur removal which is using iron sponge unit. Brief review, iron sponge is economically applied for gas containing small concentration of hydrogen sulfide, usually below 300 ppm. Iron sponge unit can not be used to remove carbon dioxide.

In this post, I want to continue the topic about iron sponge unit. More specifically about how to preliminary size iron sponge unit.

At least there are four parameters that we want to know in sizing iron sponge units:

  1. Minimum diameter of iron sponge vessel
  2. Maximum diameter of iron sponge vessel
  3. Required bed height
  4. Quantity of iron sponge
  5. Bed life of iron sponge between change out

Vessel Diameter

Why do we need to know minimum diameter of iron sponge vessel? Read More