## My First Javascript Online Calculator – Instrument Air and Compressor Sizing

For a long time, I want to post something more advance in this blog. I always want this blog like engineeringtoolbox.com or checalc.com. I want to make online calculator just like checalc.com did.

Due to my limitations, at this point I cannot create online calculator. To compensate that, I tried to make useful spreadsheet.

But, today I did it! I made my first online calculator, which is instrument air sizing calculator. Please click this link to access to instrument air and air compressor online calculator. I hope you find it useful.

It is better for you to check two of previous post related with the calculator. Calculation basis used in this calculator is based on those two post.

### BONUS!

I already replaced spreadsheet attached in this post : Spreadsheet of Air Compressor and Air Dryer Calculation. So you can trace how the calculation works.

## How to Calculate NSPH of Pump with Examples and Illustrations

NPSH of pump (net positive suction head) should be calculated/predicted in the beginning of pump sizing. The purpose is to check if the pump will work appropriately. Liquid pressure will lower right in inlet of pump impeller. If pressure reduction is lower than vapor pressure, liquid phase will change to vapor phase. The impeller rapidly collapses vapor bubbles which cause cavitation and damage.

In this post I will share you how to calculate NPSH available, which is net positive suction head calculated based on system arrangement. The value of NPSH available (NPSHA) shall be greater than NPSH required (NPSHR), which is a function of pump and to be specified by pump manufacturer.

NSPH of pump is calculated as follow:

NPSH available (ft, m) = absolute pressure (ft, m)vapor pressure (ft, m) –  line loss (ft, m) ± elevation difference (ft, m) Read More

## Natural Gas Line Sizing using General Pressure Drop Equation

Line sizing is a part of every engineering activity. Although it is not major part, line sizing is required when we prepare piping and instrumentation diagram. In this post, I want to share you how to do natural gas line sizing (actually the method is applicable for all kind of gases).

I usually use process simulator when it came to natural gas line sizing. Long time ago, I tried to size manually. But the result is not the same as from simulator. But, I tried again and yesterday I got a satisfied results.

Natural gas line sizing calculated in this post will be from manual calculation and from process simulator. API RP 14E is used as reference.

Based on API RP 14E, single-phase gas lines should be sized so that the resulting end pressure is high enough to satisfy the requirements of the next piece of equipment. So, the point is the end pressure. The velocity is also a noise problem if it exceeds 60 ft/s. However the velocity of 60 ft/s should not be interpreted as an absolute criteria. Higher velocities are acceptable when pipe routing, valve choice and placement are done to minimize or isolate noise.

API RP 14E mentioned several approach to calculate pressure drop, but in this post I will use general pressure drop equation

## Centrifugal Compressor Power Calculation

In this post I want to share you how to calculate centrifugal compressor power since there are so many process engineers are looking for it (and I also don’t know how).

To calculate centrifugal compressor power, we need these gas properties: compressibility factor (Z), molecular weight (MW), inlet gas temperature, inlet gas pressure, outlet gas pressure, adiabatic component (Cp/Cv), and mass flow rate.

Let’s start with an example while I show you step-by-step of the calculation method. In this example, we have these gas properties (I use US units).

Compressibility factor (Z) = 0.9972

Molecular weight (MW) = 18.15

Gas constant (R) = 1544/molecular weight

Inlet temperature (T1)  = 546 deg R

Inlet pressure (P1) = 15 psia (don’t forget to use absolute pressure)

Outlet pressure (P2) = 60 psia

K (Cp/Cv) = 1,274

Flow (W) = 0,184 lb/min

The required data to calculate centrifugal compressor power is head. We can use either adiabatic head or polytropic head as long as we use adiabatic efficiency and polytropic efficiency in companion. Read More

## Inert Gas Purging Requirement Calculation

Purging is an activity used to remove oxygen in piping or vessels to certain concentration, for example 2%-mole. It uses inert gas, usually nitrogen, to do that.

In this post, I want to share you how to calculate inert gas for purging activity. There are three different calculation methods used.

#### Dilution Purging

In dilution purging method, the inert gas simply flows through the vessel and reduces the concentration of unwanted component. It is used for tanks, reactors, and other vessels. Figure below is used to determine requirement of inert gas. Read More

## Predicting Holding Time of LNG in Storage Tank Without Venting

In this post, we use vacuum insulated type LNG storage tank when we don’t want to deal with boil-off gas handling. The storage tank have design pressure as high as 6-10 barg. When we store LNG in the LNG, boil-off gas will be generated due to heat leak to tank. Increasing temperature of LNG will increase total volume of LNG and vapor in the tank, thus increasing its pressure.

This phenomena leads to a simple question, “How long can LNG storage tank keep the vapor before PSV works?” Read More

## LNG Recirculation Requirement

In LNG receiving terminal, where LNG is pumped from LNG carrier to onshore LNG storage tank, we have to provide one line to intermittently circulate LNG through unloading lines. This is called LNG recirculation line or keep-cool line.  The function of this line is maintain cryogenic condition of the unloading line during holding mode. Unloading activity is conducting intermittently. Sometimes we need to wait for a week or even a month to the next LNG unloading. During this time, there is a possibility of increasing temperature in the line. To prevent that, we need to circulate LNG in unloading line. Read More

## Sizing of Compressor and Instrument Air Capacity

I want to share you how to size or select minimum capacity of air compressor and instrument air dryer. Air compressor is usually used as feed air for:

• Instrument air (for instrumentation or feed air for nitrogen generator)
• Utility air (for utility station)

Instrument Air Requirement for Instrumentation

Instrument air is required in operation of control valve and shut down valve. The rules below is used to calculated instrument air requirement for instrumentation:

• Control valve (CV) – steady state condition: 1 scfm/valve
• Control valve (CV) – transient condition: 6.25 scfm/valve
• Shut down valve (SDV) – steady state condition: 0
• Shut down valve (SDV) – transient condition: 4.8 scfm/valve