What is Pressure Drop and How Does it Affect Your Processing System?

Process piping systems are subject to a phenomenon known as pressure drop. Simply put, pressure drop is the difference in total pressure between two points in a fluid-carrying network. When a liquid material enters one end of a piping system, and leaves the other, pressure drop, or pressure loss, will occur. 

Pressure drop results from the friction caused by fluids rubbing against the piping components and the interior walls of a piping system. 

For a given system, it can be calculated with engineering models using the type of fluid, its flow rate, the piping layout and specifications (including pipe diameter), the system component specifications (such as pumps), and more.

Pressure drop in and of itself is not necessarily bad. Understanding how to calculate it in a specific pipeline allows engineers to properly design a system, and determine variables such as pipe diameter, pump specifications, and the types of valves to be used, among other things. However, there are also negative consequences associated with pressure drop if it is not well understood for a particular installation. 

If there is an excessive pressure drop in a system, the working fluid temperatures increase, and system pumps will need to work harder due to increased energy consumption. Pressure drops can also increase overall system pressure, increasing wear on components and introducing potentially dangerous over-pressure conditions. Finally excessive pressure drop can also render some piping system components inoperable due to inadequate operating pressures, or create damaging system cavitation. These negatives, and the overall impact of pressure drop, are the focus of this article.

Why does pressure drop matter?

At its most basic level, understanding the pressure drop that is associated with a specific fluid-carrying network allows process plant engineers to determine the size of the pumps/motors needed and the process pipe diameter required to move a particular type of product through a piping system.

The higher the pressure drop in the line, the greater the amount of energy consumed to maintain the desired process flow, requiring a higher horsepower motor.

Conversely, the lower the pressure drop in a piping system, the less energy consumed, providing the potential to use a lower horsepower motor. Pressure drop also determines the overall system pressure head requirements. 

Pressure head (aka pump pressure head) is simply the height to which a specific pump can lift a column of water, typically expressed in meters. 

It is essentially a measure of the force the pump imparts on the fluid being pumped. Pump pressure heads may be calculated, or may be available from the pump’s manufacturer. However it’s determined, pump pressure head must be added back into any pressure drop that’s otherwise occurring in a piping system.

If the required pressure head is too large as a result of the need to overcome a large pressure drop, this can have an adverse effect on components within the system, including the proper operation of ancillary equipment, premature failure of seals, and potentially harmful over-pressure situations.

The effect of pressure drop on seals

Seals used in equipment such as pumps and heat exchangers have specific pressure limitations. When the equipment is operating within a suitable range (in terms of pressure, temperature, speed, etc.), seals will have a pre-defined life cycle. 

When equipment is pushed to operate outside the optimal range, caused by factors such as over-pressurization, the seals will degrade or deform, causing leaks in the system. 

Even after the overpressure incident has been corrected, the seals will continue to leak since they no longer fit properly.

The effect of pressure drop on safety

Over-pressure situations caused by pressure drop can also lead to safety concerns. Processing systems are designed to operate safely and efficiently. When system piping is undersized for a specific application, the pump has to be oversized to accommodate the pressure drop. In this situation, equipment close to the pump experiences higher-than-acceptable pressures. 

This can lead to ruptures in the piping, exposing processing plant personnel to unsafe working conditions (e.g., hot liquid products, corrosive cleaning chemicals, etc.)

What Affects Pressure Drop?

1. The Product

When considering the potential for pressure drop in a specific fluid processing system, the first thing needed is an understanding of the nature of the product being pumped through it. 

Fluid properties including

• Density
• Heat capacity
• Temperature
• Viscosity 

all affect pressure drop. 

For example, in a food processing plant, some products—such as ketchup—will change their viscosity dramatically when being pumped through a pipeline due to shear. These types of products will become thinner due to friction caused by the passage through the pumps and the interior surfaces of the pipes. 

This phenomenon is called thixotropy, which is a time-dependent shear thinning property. Thixotrophic fluids are normally viscous when in a static state (such as ketchup in a bottle), but become thinner or less viscous when shaken or agitated, returning to their normal state when the source of agitation is removed.

In contrast, other products, such as vinegar, act more like Newtonian fluids under processing conditions. Newtonian fluids are liquids which are not thixoprophic, and are not subject to changes in viscosity when subjected to shear force. Products which exhibit Newtonian characteristics, therefore, can contribute to higher pressure drop when being pumped through a pipeline because their viscosity does not significantly change as it passes through the system.

2. Mechanical Components

Mechanical components in a piping system--including valves, flow meters, adapters, couplings, and tubing--can also have an impact on pressure drop. Apart from pumps, all of these components commonly found in a process piping system will contribute to a system’s pressure drop because they remove energy from the process flow, rather than adding to it. 

The mechanical pressure drop also depends on 

• The cross-sectional area of the pipe
• The pipe's interior surface roughness
• The length of the pipe
• How many bends there are in the system
• The geometrical complexity of each component

For example, changes in fluid flow or direction--such as those created by introducing 45- or 90-degree elbows-- can increase friction and pressure drop. Also, the greater the distance that the fluid must travel in the system, the greater the surface area there is to cause friction.

3. Changes in Elevation of Piping

Pressure drop can also be significantly affected by a change in elevation in the piping system. If the starting elevation of a pipe is lower than its end elevation, there will be an additional pressure drop in the system caused by the rise in elevation (measured in terms of fluid head, which is equivalent to the rise in elevation). 

Conversely, if the starting elevation of the pipe is higher than its end elevation, there will be an additional pressure gain due to the drop in elevation (again, measured in terms of fluid head, and equivalent to the fall in elevation in this case).

For a specific piping system, the overall pressure drop may be calculated by applying several equations. One example used to calculate the pressure drop in the process piping is given by the following:

P(end)= P(start) - friction loss- fittings loss -component loss + elevation (start-end) + pump head

Where

• P(end)= pressure at the end of the pipe
• P(start)= pressure at the start of the pipe
• Elevation (start-end) = (elevation at the start of the pipe) – (elevation at the end of the pipe)
• Pump head= 0 (if no pump is present)

So, when designing a process system to minimize or eliminate pressure drop, process plant engineers should do the following:

1. Ensure that the process pipe’s interior diameter and the size of the pump (horsepower, throughput) are properly sized for the type of fluid that’s being piped through the system. Mistakes made in either of these can result in either excessive pressure drops or overpressure situations.
2. Minimize the number of additional mechanical components (valves, flow meters, adaptors, and couplings) in a process pipeline, as all of these can add to problems with pressure drop.
3. Ensure that the process pipeline is laid out to be as compact as possible, minimizing pipe lengths and bends. Excessive pipe run lengths and changes in direction will contribute to pressure drop.
4. Make sure that the process pipelines are as level as possible, ideally with both staring and end elevations close to the same height. As noted above, changes in piping elevation in the overall system will contribute to either pressure drop or overpressure situations.