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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

Classification of Capital Cost Estimation

Based on my experience, in almost every stage of project, capital cost estimation is always part of document deliverable. During feasibility study, Pre-FEED study, or during FEED (Front-End Engineering Design), capital cost estimation is always created. But there are several degrees of capital cost accuracy between those studies. In this post I want to share classification of capital cost estimation.

I believe that capital cost estimation is more art than science. The more experience a cost estimator gets from cost estimation, the more accurate the estimation. In many cases, an estimator must use considerable judgment in preparing the estimates.

Classification of Capital Cost Estimation

There are at five types of fixed cost capital estimates:

  1. Order-of-magnitude (ratio estimate). It is rule-of-thumb methods based on cost data from similar-type plants are used. The probable accuracy is -30% to +50%
  2. Study estimate (factored estimate). This type requires data such as preliminary material and energy balances and major equipment items. The probable accuracy of -25% to +30%
  3. Preliminary estimate (budget authorization estimate). This type needs more details about the process and equipment, such as design of major plant items are required. The probable accuracy is -20% to +25%
  4. Definitive estimate (project control estimate). The data needed for this type of estimate are more detailed than preliminary estimate and include preparation of specification and drawings. The probable accuracy is -10% to +15%
  5. Detailed estimate (firm estimate). To prepare this type of estimate, complete specification, drawings, and site surveys for the plant construction are required. The probable accuracy is -5% to +10%

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Gas Sweetening using Iron Sponge Process

Iron sponge process is economically applied for gases containing small concentration of hydrogen sulfide (H2S), usually less than 300 ppm, operating from low to moderate pressure in a range of 3.45-34.5 barg (50-500 psig). Iron sponge process cannot be used to remove carbon dioxide.

Iron sponge process is the oldest and still the most widely used batch process for sweetening of natural gas and natural gas liquids. Overall, iron sponge process has the following characteristics which make it still attractive to be applied: simple process, low capital cost, and relatively low chemical (iron oxide) cost. Furthermore, pressure has a relatively little effect on its adsorptive capacity of a gas sweetening agent. Read More

Firefighting Foam Demand Calculations

Firefighting Foam has been widely used as a fire extinguisher for flammable and combustible liquids for many years. It is consisted of premix solution (a ratio of water and foam concentrate) mixed with air. It forms a stable form blanket which spreads above the fuels and adheres to surfaces.

Foam forms a film above the fuel surface and seals vapor. Fire fighting foam concentrate prevents fuel contact with oxygen, resulting in fire suppression. It also has a high cooling effect and will avoid reignition. Read More

Inert Gas Consumption for Tank Blanketing Calculator

In this post I want to share calculator to estimate inert gas consumption for tank blanketing. Please be noted that the result is average inert gas consumption. For peak consumption, please check my previous post.

I think this is the second time I created calculator. I am quite satisfied with the results because I used plugins.

I hope you find this post useful.

Inert Gas Consumption for Tank Blanketing

Inert gas is used to blanket certain fixed-roof tanks for safety. Nitrogen is a common inert gas used for the purpose. In addition to safety of storage tank, the benefit of tank blanketing includes improved product quality and equipment life cycle.

Why nitrogen is commonly used in tank blanketing?

Nitrogen has inert properties, wide availability, and relatively low cost at any economic efficiency. Other gases, such as carbon dioxide or argon, are also sometimes used for certain application.

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