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Introduction

We all try to avoid making mistakes. In contrary, we often learn from our mistakes. Making mistakes when someone is doing something for the first time is quite possible and expected but I have seen experienced engineers making careless mistakes on some of the most commonly used terms and concepts that are either perceived to be very simple or considered to be well understood by people after few years in our industry.

This paper highlights some of these common mistakes and the terrible outcomes of such misconceptions or miscalculations that I have across over the years.

Gas flow rate measurement unit at standard conditions

Using MMSCFD (million standard cubic feet per day), MMSCMD (million standard cubic meter per day), BCMA (billion standard cubic meter per annum) or Nm3/h (normal cubic meter per hour), are among some the most commonly used terms in specifying the capacity of unit.

One of the most common misconception about this unit of measurement is that it is wrongly taken as the unit of volumetric flow. In a typical example, one had taken the capacity ratio of two gas compression units to estimate the cost of new unit using the 0.6 power equation (cost ratio = capacity ratio ^ 0.6). The capacity of unit in MMSCFD had been used to calculate the capacity ratio which resulted in a significant error in the calculated cost for the new unit.

MMSCFD is the volume flow of gas at standard conditions (14.5 psia and 60˚F - Note 1) - not actual conditions. In other words, the effect of temperature and pressure and molecular weight have to be applied on the MMSCFD to be converted to the actual volumetric flow before being used in the above equation.

For two identical gas compression units with the capacity of 100 MMSCFD, and the same inlet gas molecular weight and temperature, the one with 10 bara suction pressure will have the actual volumetric flow rate 2 times higher than the one with the suction pressure of 20 bara which makes it 1.5 times more expensive than the other. Not a small error!

If we ignore the effect of molecular weight, the flow rate of gas in the standard conditions is more a mass flow rate than the volumetric flow.

Each 22.4 Nm3 is 1 kgmol of gas corresponding to MW in kg (Note 2)

Each 380 SCF is 1 lbmol of gas corresponding to MW in lb

Therefore, 100 Nm3/h of gas with molecular weight of 44.8 kg/kgmol is equal to 200 kg/h. Similarly, 100 SCF/h of gas with molecular weight of 38 lb/lbmol is corresponding to 10 lb/hr.

Notes:

1.    Different sets of pressure and temperature are used for the standard conditions in the different parts of the world

2.    Normal conditions are 1 atm and 0˚C.

Part per million (PPM)

Making errors while reporting the concentration of a component in ppm unit is most probably one of the most common mistakes in the oil and gas industries.

There are two main reasons for this:

1-    ppm is used to specify the concentration of a gaseous component in a gas or liquid mixture, a liquid component in the liquid mixture or gas, and last but not the least, solids in the gas and liquid phases which makes it hard for the user to interpret the definition.

2-    ppm comes with the v (volume), mol (mole) and w (weight) suffixes which makes the conversion from one unit to the other a bit tricky. Needless to say that coming across ppm without any of these prefixes is not uncommon either! (which adds up to the confusions).

First of all, let’s agree that using ppm without suffix is wrong and not repeat this mistake again.

In order to be able to correctly interpret the ppm definition and convert it to other units, we need to know a little about the measurement method. For example, for solid in liquid, the composition is most probably specified by filtrating the suspended solids from the liquid. In this method, the weight of solids in mg divided by the volume of liquid in m3 is often reported as ppm_v. A GC or a moisture analyzer measures the components in volume fractions. I don’t know of any laboratory apparatus that measures the concentration in mole fraction. ppm_mol has become popular basically because the commercial software packages report the composition in mole.

When the concentration of a liquid component in a liquid mixture is reported, the volume of each phase is often measured. Therefore, the volume of one component is divided by the total volume of mixture and reported as ppm_v.

The following table shows the default definition of ppm in the oil and gas industry based on the phase of component and mixture:

Now, you are ready to convert ppm from one unit to another:

ppm_w of liquid component A in a liquid or gas mixture= ppm_v of component A x density of component A / density of liquid or gas mixture

ppm_mol of liquid component A in a liquid or gas mixture = ppm_w of component A x MW of liquid or gas mixture / MW of component A

For a gas mixture, ppm_mol is always equal to ppm_v.

It should be noted that the difference between the ppm_v and ppm_w and ppm_mol can be very considerable. For example, for a liquid mixture with the following properties:

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MW of liquid component A (say average MW of sulfur species) = 80 kg/kgmol

MW of liquid mixture = 40 kg/kgmol

Density of liquid A = 500 kg/m3

Density of liquid mixture = 1000 kg/m3

If the concentration of component A in the heat and material balance is 100 ppm_mol, it is equal to 200 ppm_w and 400 ppm_v. So, imagine the design and cost impacts of reporting the concentration of sulfur components in the fired heater's liquid fuel in ppm_w when you actually mean ppm_mol. A horrible mistake!

While it is pretty common for liquid and gas phase concentrations to be quoted in all ppm units (i.e. ppm_w, ppm_v and ppm_mol), the concentration of solids in gas and liquid is often measured and reported in ppm_w (mg/kg) or ppm_v (mg/m3).

ppm_w of solid component A in a liquid/gas mixture in mg/m3 = ppm_v of component A in mg/kg x density of liquid/gas mixture in kg/m3

Excessive Overdesign to cover for the data uncertainties

It is pretty common for a conceptual or FEED designer to add some margins on the process parameters for sizing an equipment or unit. This is to ensure that the design and cost estimate are conservative when the input data are not certain and/or vendor information is not available.

Although, the general understanding is that having a larger equipment than required is a better approach than buying an equipment which fall short of normal duty. However, excessive overdesigns can have very destructive effects on the equipment, endanger safety of plant and people and lead to underperforming equipment, energy loss, excessive maintenance, narrow operating range in addition to the higher capital purchase cost.

So, the expectation is that the detailed engineering designer to carefully look at these margins added in the different phases of project and adjust the design specifications of purchased equipment with respect to final design data and vendor information. Although this is not always possible but the lack of knowledge about the consequences of using large overdesigns also greatly contributes in the failure to take this requirement into account.

Some of the most well-known cases where excessive overdesign can cause problems are:

Relief valve: oversizing relief valves can lead to chattering in low flow scenarios. The resulting vibration can cause misalignment, valve seat damage and even mechanical failure of valve internals and associate piping. Read about a related incident here :

Pumps: oversizing the pump capacity can push the pump off the best efficiency point (see below figure) leading to higher power consumption, increased maintenance and higher ongoing running costs. Operating at excess capacity can require a higher NPSHr, cavitation leading to reduced efficiency and premature component failure as well.

Read more here:

Control valve: an oversized control valve operates at very low opening during the normal operation resulting in loss of controllability. Read more here

One of the cases, I came across recently was a vertical fuel gas KOD which was provided with the demister pad to separate fine liquid particles from the fuel gas to meet the liquid carry over specification of 0.1 gal/MMSCF. The fuel gas KOD diameter was selected as 1000 mm (area 0.78 m2) for the access requirement. The designer calculated the mesh pad surface area of 0.3 m2, but thought it would not be harmful to install a larger mesh pad. So, the datasheet was issued showing the entire cross-sectional area of the vessel covered with the wiremesh mist eliminator. Therefore, the mesh pad area was more than 150% overdesigned.

The designer overlooked the fact that the performance of wiremesh is a velocity dependent parameter and vendor K value for demister is valid for the 30% - 110% range of the design flow rate. With the proposed design, the actual velocity during normal operation was 38% of the design velocity. This meant that the vessel would start to underperform when the flow rate reduced below 80% the normal capacity of plant. The separation efficiency at the turndown capacity was significantly lower than the required. While the correct design should have considered a mesh pad with the diameter of 600mm covering the gas outlet nozzle as shown in the following figure. A carless design!

 I don’t know of any process equipment that will not be adversely affected by the excessive overdesign.

A request to readers of this blog

If you have come across similar mistakes, please share it in the comment section of this note for the benefit of other readers. Thank you!

About the Author

Saeid Rahimi Mofrad is a principal process consultant with a proven track record in the hydrocarbon industry and a wide experience on the equipment sizing and selection and flare system design. He has prepared many sizing procedures and more than 180 spreadsheets for different process calculations. He has also published and presented more than 50 papers in different international magazines, websites, and seminars and conducted many training courses and workshops for his employers as well as third parties.

Mr. Rahimi is the founder of Chemwork discussion forum on LinkedIn (http://www.linkedin.com/groups/Chemwork-3822450) and has also developed a professional process engineering software called Chemwork for oil and gas processing equipment sizing and design calculations in a very user-friendly environment. Software demos are available on YouTube (https://youtube.com/user/Chemwork).

He can be reached at s.rahimi@enernik.com

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Share 44 9 Comments Goussas Mohamed Nasreddine

PROCESS ENGINEER - Oil and Gas Field - Engineering

3y

• Report this comment

thanks so much for this sharing

Like Reply 1 Reaction Mojtaba Habibi

Senior Process Engineer at Worley

3y

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Dear Saeid, Perfect write-up as ever. Let me share my real experience for one of recent projects (although these are related to inadequate overdesign margins): 1. Air blower for flare stack smokeless operation: As per API 521 this is recommended to consider capacity equal to 10-30% of stoichiometric air demand. For my case air blower was sized based on 10% ratio. In reality a long term headache has been experienced with flare smoky operation. 2. First stage separator in upstream oil production units which involves multiple producer wells along with gathering manifolds has been subjected to unstable operation and slugging issues which even could not be properly assessed with high level flow assurance studies. For both of above cases there was almost no room for modification/corrective action.

Like Reply 2 Reactions 3 Reactions Manikandan Murugaiyan

Senior Process & Facility Engineer - Gas Processing, LNG & Sulphur Complex

3y

• Report this comment

I have seen confusion happening sometime in indicating the liquid u seal requirements for tank overflow and other similar application requiring head to be maintained in the seal leg.

Like Reply 1 Reaction 2 Reactions Omar Bin Zia

Process Engineer (Oil & Gas- Process Design, Technical Services and Projects)

3y

• Report this comment

Saeid Rahimi Mofrad , great piece as usual. Nice to see you back writing technical stuff after so long, I hope this will be a stepping stone in a long series of technical articles. Keep enlightening

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