Master Pipe / Line Sizing As a Process Engineer

There are various types of flow regimes for a two phase flow. It can be :

Wave flow

Stratified flow

Bubble flow

Annular mist flow

And slug flow which is the most dangerous one.

Slug flow is undesirable as it can cause vibration and severe damage in the pipe

How to calculate inside pipe diameter from nominal diameter and pipe schedule?

How would valves, elbows, tees and other fittings affect the pipe equivalent length?

To calculate the pipe size, you can use the equations used to calculate velocity and pressure drop. For two phase flow, more complex equations are used.

I shall give here some excel sheets used, you can also use some online tools as Checalc.com or a mobile app as Process Engineer Toolbox which is available on IOS and Android.

This is an example of a performance curve for a centrifugal pump provided by the pump manufacturer. It represents the flow versus head curve.

As we notice in the curve that as the pump flow increases, it will give less head which means less pressure. This shall happen when the resistance at the pump discharge decreases, may be the control valve opens or less destination pressure or for whatever reason.

We can determine the expected operating point when plotting the system curve showing the static and dynamic losses.

The static losses simply represent the maximum elevation at pump discharge which is a constant head, so let’s add it here.

What about the dynamic losses? These shall comprise the pipe losses, control valve pressure drop and other equipment or flow element losses, these shall directly depend on flow rate, their curve will look like this.

As we see here, we can get the dynamic losses, adding them to the static head, we can get the total head losses.

And here we get the operating point

So what if we want to to change the operating point? Let’s close the control valve for example, this means that dynamic losses shall increase. So the curve shall look like this.

And this shall be the new operating point at a less flow rate. If we want to increase the flow rate, let’s open the valve, so the curve shall look like this and we can get larger flow rate as the system resistence decreased..

The control valve inherent characteristics provided by the valve manufacturer are relative to the valve only.

But in the actual case, the valve is not alone, there is a system upstream the valve and a system downstream the valve.

So here comes an important concept which is the control valve authority. This represents the fraction of control valve pressure drop to the total system pressure drop.

We can see it in the this equation.

So when talking about the inherent valve characteristics, it is considering the whole pressure drop is only in the valve, which means that the valve authoity is 1.

But when we put the valve in the system, we have lower pressure at the valve inlet, this is because of there is a pressure drop in the upstream piping or equipment.

Also because there is a pressure drop downstream the valve, this means that there will be a higher backpressure on the valve.

So we will have less pressure drop to be exerted by the valve and depending on the flow rate, the losses in the upstream and downstream will vary, which will result in the variable pressure drop across the valve. So in this case which is the actual case as no one installs the valve alone, the valve authority is less than one

Let’s even explain it in deep, we have a control valve on pump discharge.

So we have a pump curve where the pump head decreases as the flow increases.

We have control valve upstream piping pressure drop and downstream piping pressure drop.

Now let’s plot the pump curve.

Then let’s plot the head losses in the upstream and downstream piping on the curve.

So at a low pump flow, the pump gives us higher head, and at the same time, we have low system losses. So we shall need the control valve to exert a high pressure drop. At low system resistance, the control valve authority will approach to one.

But when we need a high flow, the pump shall give us less head, and the system losses will increase. This will lead to a low valve pressure drop and the authoritiy will drop.