The selection of pump technology can be a substantial factor in system design for the numerous industries under pressure to increase productivity and quality standards, improve throughput, and reduce energy costs. This is particularly important in markets such as oil and gas, municipal water and wastewater, pharmaceutical, chemical, and power generation. Let’s take a closer look at a key technology which can help drive system efficiency in several areas: positive displacement.

What is a positive displacement pump?

Positive displacement pumps are designed to capture a certain amount of fluid and/or displace it in one direction. These pumps have a very specific action – to push fluid out in a

controlled amount for accurate dosing or treatment. By design, they are less susceptible to flow

changes based on system pressure. This increases the level of accuracy and dependability of

flow rates.

Positive displacement technology differs from other pump technologies such as centrifugal

pumps. A centrifugal pump’s reliability diverges with increased pressure. The discharge side

of the pump assembly begins to pump less fluid until it gets to the point where it can’t pump

against the pressure and it simply recycles the fluid. A centrifugal pump’s purpose is primarily

as a mover of fluids, but is not generally tied to repeated controlled injection, whereas a positive

displacement pump will always continue operating at the same volume.

Types of Positive Displacement Pumps

Positive displacement is used to describe a large group of different pumps, which fall into three

types of operation:

• Rotary: fluid is continuously moved when the rotary or gears are rotating
• Reciprocating: a piston, plunger, or diaphragm moves liquid into the pump reservoir where inlet and outlet valves control the amount of fluid entering as well as the discharge rate of fluid
• Linear: a piston in the cavity moves fluid in a straight line

A Deeper Dive into Linear Pumps

One type of a linear pump is a rope pump. It is simple in design and construction, allowing

for a low-cost solution, generally used for water-based applications. It is also easily

powered by alternative energy sources like wind or solar power. While the simplicity of

this design allows for simple use and maintenance, it limits the ability to work with high

volumes or in high pressure applications.

Rotary Examination

Peristaltic pumps, a type of rotary pump, are often used in medical applications or

laboratories, as they can be miniaturized specifically for these devices. When the pump is

new, it effectively meets the requirements of a positive displacement pump – pumping at

a constant flow rate. However, this type of pump relies on an elastomeric tube, which can

wear at different spots and cause changes in flow rates. This makes it a less reliable option

for long term use and harsh applications.

Reciprocating Example

Various metering pump manufacturers use a piston or a plunger that pulls fluid in and pushes out a specific amount of fluid at a certain rate based on the pump’s settings. The metering pump is a positive displacement chemical dosing device with the ability to vary capacity manually or automatically as process conditions require.

It features a high level of repetitive accuracy and can pump a wide range of chemicals

including acids, bases, corrosives and viscous liquids or slurries. The pumping action is

developed by a reciprocating piston which is either in direct contact with the process fluid

or is shielded from the fluid by a diaphragm. Metering pumps have the integrated ability

to change capacity and can also apply variable speed to increase the pump’s turndown

ratio. This is slightly different from other positive displacement pumps which can only use

variable speed tech to change flow. The technology used in metering pumps is best for a

controlled rate, repeatable accuracy, and linearity. This feature makes this type of pump the

best alternative for applications that need to inject a specific dosage of various chemicals

that include a wide range of characteristics in pH, viscosity, solids content, specific gravity,

and other factors that make them difficult to handle.

Processes requiring such chemical dosage include:

• Downstream, upstream, and midstream oil and gas
• Water and waste treatment
• Chemical manufacture
• Industrial water
• Pulp and paper

Metering pump technologies are well suited for environments that require fluctuating flows,

such as water treatment plants and oil and gas applications. Using signals it receives,

the metering pump will respond automatically to change the flow of the chemical being

injected into the water.

Create an Optimized System for Your Application

System Overview – what goes into a system?

The key to success, regardless of the type of pump you use, is that the system must be

designed strategically so that everything functions effectively. A system refers to everything that

makes your process function, from where the tank is holding the chemical to the point where

it’s injected. This means there are multiple points of consideration: tank, chemical selection,

injection point, piping, pumps, measuring devices, flow meters, and the various accessories that

are used.

Designing the “right” system can lead to increased dependability and decreased cost of

ownership. However, companies can struggle to pick all the right pieces. There are so many

ways to build a system that can work, but few options for the most efficient solution.

For example, peristaltic pumps have recently found use in place of a metering pump for municipal applications because the pump can be very effective when dealing with chemicals that off- gas, such as sodium hypochlorite.

A system designed with this pump may have acceptable performance; however, the tubes require constant maintenance or replacements, thereby increasing the cost of ownership. And as the tube wears, its ability to maintain the same capacity is compromised. Metering pumps can be just as successful pumping off-gassing liquids if the pump and system are designed properly to avoid gas binding. When properly applied and used in a well-designed system, metering pumps will far outlast peristaltic pumps and provide maintenance-free operation

for a significantly longer period.

Generally speaking, pumps are designed to fit their specific intended applications. Metering

pumps are a slight exception to this since they can leverage positive displacement technology

to handle everything from low pressure to high pressure and all different types of fluids.

Additionally, manufacturers of highly customized metering pumps can offer a unique solution —

increasing accuracy and efficiency. Possible areas of customization or modification include:

• Multiplex pumps on a common motor
• Unique wetted materials
• Modifications for specific liquid
• Response to various process signals
• Inclusion of process sensors

Considerations for Positive Displacement Pumps

Accuracy

As discussed in the introduction, positive displacement pumps serve an important

purpose, so accuracy and consistency are critical regardless of pressure changes.

Understanding the pump and how it achieves accuracy is important. For example, with

metering pumps, one gets a repetitive amount that one can measure and that one can count

on with every stroke of the piston.

Footprint

Footprint, or size of the pump, is sometimes a consideration and is usually based on the

application. For example, if one is searching for a solution for offshore oil or an oil platform –

every square foot of your operation will cost much more than the space required in a field

for a water treatment plant.

Chemical Requirements

The type of pump that’s purchased or how it is customized is largely based on what type of

chemical, its corrosiveness, viscosity, and how accurate the dosing of the chemical needs

to be. For example, polymers can be a difficult material to pump. A facility may choose

a peristaltic or a progressive cavity pump based on design or they could optimize their

system to suit a metering pump to handle the flow requirements.

Flow Requirements

Positive displacement pumps can be used for a wide range of pressure requirements

(10 psi through 30,000 psi) due to their ability to keep flow consistent regardless of

changes in pressure. Pumps catering to low pressure (50 or 100 psi) are typically best

for water treatment applications such as municipal water plants and wastewater plants.

While the pressure tends to be low, the amount of variability of flow is often very high;

this is referred to as turn down.

Applications such as offshore oil and gas, for example, are pumping gases like

methanol down to the oil well site to keep it from freezing. In this process, pumps are

required to operate effectively despite the high well pressures, which become even

higher as you go deeper offshore. These applications can reach the 20,000 plus psi

range.

Efficiency

There are two types of pump efficiency: electrical and hydraulic. Electrical efficiency

tends to be a big focus for the automotive market. The pump industry sees a lot of

demand from motor manufacturers to create motors with maximum efficiency to reduce

energy costs.

Hydraulic efficiency is directly tied to the type of pump chosen. Certain pumps have

leak through between pieces of the unit, such as the liquid end of the pump gear. If an

application requires high efficiency; buyers will want to avoid a plunger pump or a pump

with no diaphragm seal inside of it to prevent hydraulic loss. High-pressure applications,

in particular, are subject to more loss of hydraulic power because the increased flow

causes more leaks. In these cases, facilities need to look very closely at the theoretical

flow and how it is impacted by pressure to understand efficiency. Metering pumps offer

80-90% efficiency rates at standard conditions, making them an efficient option even

under high pressures.

Trends in Pump Technology

Keeping up with increased demands in size reduction, performance, efficiency, and

environmental protection continues to shape how today’s products are designed, and

positive displacement pumps are no exception. Here are just a few ways in which pump

technology continues to evolve.

Communication

One of the most influential forces shaping product development is the

ability of the pump to communicate. The basic needs of a pump, such as certain flows and

pressures, are going to change with every application, but there is an increased demand for

a pump that can talk back for better process control. Plants are operating with fewer and

fewer people; in some cases, they’re operating with no personnel onsite at all. The number of

pumps that have been deployed that respond to an automated control signal are much

higher than a few years ago, and this trend continues to grow.

Sealing Dependability

Diaphragm technology is ideal for those looking to address environmental and efficiency concerns, since it’s designed to prevent leaking. This technology has evolved in positive displacement pumps and in metering pumps in particular.

While the dependability of the sealing action has increased, there have also been additions

of other technologies to support the process, such as diaphragm leak detection systems.

This helps teams know if the diaphragm is failing and when maintenance is required. Having

the system issues a warning before total failure helps to prevent the fluid from leaking outside.

Detection systems are also essential to the hydraulic efficiency of a pump, as any connection

could potentially be an additional point of hydraulic loss. As this design has evolved, it

has allowed manufacturers to create a more efficient pump and that directly influences the

performance of the product.

Each positive displacement pump brings applications unique advantages and this technology continues to evolve. Plant managers should carefully consider accuracy, footprint, chemical and flow capabilities, and efficiency of each pump before selecting the right option for any system. If you’re not sure what the best fit for you is, contact Falcon Project Consultants.