The fluid flow process is one of the five major unit operations in the chemical industry. Fluid flow processes include transfer, filtration, solid fluidization, and transport activities. Centrifugal pumps have traditionally been a popular standard in the fluid flow process industry, but in some cases, these pumps have been inefficient and costly. In this case, a positive displacement (PD) screw pump can be a viable alternative. PD screw pumps can be used to handle a variety of fluids, including those with higher viscosity.
There are many benefits to using screw pump technology in chemical fluid operations, including the ability to handle a wide range of flow rates, pressures, liquid types and viscosities, and the ability to maintain consistent flow rates even in the presence of backpressure. To understand in more detail the benefits of screw pumps compared to centrifugal pumps, let’s first look at how screw pumps can improve chemical process applications.
Screw pumps have their own unique advantages
In the field of industrial manufacturing, unit operations generally refer to the basic steps of the entire process. In the chemical industry, the concept of unit operations is broader, encompassing a series of production processes that must be completed accurately in order to process various raw materials into useful finished products. There are five types of unit operations in the chemical industry. The focus of this article is on fluid flow processes. It generally includes transfer, filtration, solids fluidization and conveying activities.
(The other four unit operations are heat transfer (evaporation and heat exchange), mass transfer (distillation, extraction, adsorption and drying), transmission (refrigeration and gas liquefaction) and mechanical (crushing, crushing, screening and screening) processing.)
For many years, centrifugal pumps have been regarded as the standard pumping technology for fluid transfer applications in chemical processes. In all fairness, centrifugal pumps do meet the basic needs of operators, as the methods make them well-suited to the high-volume transfer conditions common in industrial applications. Centrifugal pumps also handle thin, water-like fluids well. These fluids must be transported through a network of pipes with variable flow rates.
The focus of this article is to compare centrifugal pumps to illustrate the advantages of different types of pumping technology, including positive displacement twin-screw and triple-screw pumps, in critical fluid operations in the chemical industry, many of which involve abrasive conditions. , corrosive and hazardous fluids.
The biggest challenge in making progressive cavity pumps widely used in the chemical industry is that operators are no longer obsessed with centrifugal pumps as the only option for their fluid transfer jobs. In many cases, chemical systems revolve around pumping technology, not the other way around.
This means that engineers first become familiar with centrifugal pumps and want to incorporate them into a category that matches the operation of their systems.
They understand how centrifugal pumps work, and their advantages, and firmly believe that this type of pump is the best technology for the system they want to implement.
Based on this understanding, design engineers will design a system for mixing or heating chemical raw materials or composite materials, and control the process to reduce the fluid viscosity to 300 centistokes (cSt) to facilitate centrifugal pump processing. This essentially changes the state of the fluid to adapt it to the pumping technology, regardless of cost implications.
Although it is possible to change the state of the fluid to meet the operating requirements of a centrifugal pump, operators still need to ensure that the centrifugal pump is operating at or near the best efficiency point (BEP).
But in reality, centrifugal pumps can rarely operate in BEP because it is difficult to achieve a pure pumping state. In view of this, it is generally believed that centrifugal pumps will operate within the optimal efficiency window of 80% to 110% BEP.
If the operating conditions of the pump deviate too much towards both ends of the BEP, the pressure on the impeller will become uneven, resulting in an increase in radial thrust and deflection of the pump shaft. In this way, the bearings and seals will bear greater loads, causing damage to the pump casing, baffle and impeller.
While the chemical industry has long used centrifugal pumps to handle fluids, screw pumps can accommodate fluids of varying temperatures and viscosities. This means operators don’t need to spend time and money manipulating the fluid transfer process to match the pump’s requirements.
In addition, meeting production speed and quotas is the top priority of chemical plants. As operating costs continue to rise, operating efficiency also needs to increase in order to optimize energy consumption during pump operation.
In this regard, centrifugal pumps have shortcomings:
1) Usually engineers will choose an oversized pump to reduce the complexity of pump selection, which will lead to increased operating and maintenance costs, offset operations, and energy consumption that is easily higher than actual demand.
2) During the operation of the centrifugal pump, the flow rate will decrease as the pressure increases. Therefore, jobs that require stable flow rates and high time costs may take longer and therefore cost more. Centrifugal pump performance can be negatively affected when pumping fluids with viscosities above 100 cSt.
3) These inefficiencies will lead to increased energy consumption and may even sacrifice production efficiency, thus fundamentally having a negative impact on the operation of chemical process processes.
Another option is a positive displacement (PD) screw pump. The difficulty in increasing the use of screw pumps in the chemical industry is two-fold: the natural advantage of the large installed capacity of centrifugal pumps, and convincing design engineers that alternatives such as screw pumps are available.
Many engineers also hold the view that screw pumps can only handle low-flow fluids.
In fact, today’s screw pumps have significantly improved pumping flow rates, ranging from 220 to 11,000 gallons per minute (gpm), equivalent to 833 to 41,635 litres per minute (L/min). This range has become increasingly common.
PD screw pumps are designed to handle a variety of fluids, even high-viscosity fluids. The screw pump operates in such a way that the screws in opposite directions mesh with each other and form a sealed cavity with the surrounding pump casing.
As the drive screw rotates, fluid begins to move steadily and continuously toward the pump’s discharge port, producing a consistent volumetric flow rate regardless of pump pressure.
Progressive cavity pump technology brings many advantages to chemical fluid transfer applications, including:
1) Ability to handle a variety of flow rates, pressures, liquid types and viscosities
2) Stable flow, even if there are different back pressures caused by viscosity changes
3) High volume and overall operating efficiency, reducing operating costs
4) Output or capacity can be controlled through a higher pump speed ratio
5) Low internal speed
6) Self-priming operation and good suction characteristics
7) Low mechanical vibration, extending service life
8) Inherently smooth and quiet operation characteristics
9) Extremely low pulsation, reducing stress and extending the life of related fluid delivery components (pipes, hoses, seals, bearings, etc.)
No one unit of the chemical process is more important than another, but if manufacturers want to optimize the performance of their fluid transfer applications, it would be wise to consider the advantages that positive displacement progressive cavity pumps have to offer.
Progressive cavity pumps may cost more to purchase than centrifugal pumps, but remember the old saying: “Pay now or pay later.”
In addition to cost, the main operating advantages of screw pumps include the ability to handle fluids with a wider viscosity window at higher speeds and pressures, with low energy consumption and no need to consider BEF.
These advantages allow the screw pump to make the fluid transportation process of key chemical processes more efficient, reliable, economical, and adaptable.