Optimizing Centrifugal Pumps: A Blueprint for Chemical Processing Success

One of the most important aspects of pump operation is its interaction with upstream and downstream components. This interaction significantly affects the pump's performance, reliability and overall operation. Unfortunately, this aspect is often overlooked, leading to poor performance, operational issues and reduced reliability. Even if the highest-quality pump is selected for an application, neglecting its interaction with associated systems can result in operational problems, damage and failures.

For example, issues can arise when the system forces the pump to operate far from its best efficiency point (BEP) for long periods or when poor suction piping configuration leads to cavitation.

The following discussion focuses on centrifugal pumps in chemical processing plants. They play a crucial role in chemical plant operations due to their ability to handle corrosive liquids. Selection requires careful consideration of configuration, types and sizing.

Pump Selection and Sizing: Matching Flow and Pressure Requirements

Pump sizing involves matching the flow (capacity) and pressure (head) rating of a pump with the flow rate and pressure required for the process or unit. Achieving the required flow rate requires a pump that can generate a high enough pressure to overcome the hydraulic resistance of the entire downstream system, including piping and equipment.

The system head varies proportionately to flow rate. Using the system curve, we can demonstrate the relationship between the flow rate and hydraulic resistance for a particular system (1). The pump sizing usually involves matching the required outlet pressure of a pump, whose output flow varies nonlinearly with pressure, with the pressure needed for the given system, which varies nonlinearly with flow. Therefore, this is usually a highly nonlinear problem to solve.

To achieve the highest efficiency possible, the specified duty/rated/operating point(s) should be close to the best efficiency point — the pump’s BEP. Suitable selections would have the duty point no less than 65% of the flow rate at the pump’s BEP or greater than 115% of the flow rate at the BEP.

The pump should have a net positive suction head required (NPSHR) at least 2m less than the net positive suction head available (NPSHA). Alternatively, NPSHA should be at least 133% greater than the NPSHR criteria at any point between the zero-flow (shutoff) and the maximum operating flow attainable against the characteristic curve. The pump's NPSHR should be based on the actual 3% head drop method test result.

The pumps should be stable in operation at all heads and under all conditions of single or parallel operation (in case of two or more pumps in parallel operation), within the full range of intended operation from the maximum head to the minimum head. Pumps should have constantly falling head-flow curves between zero and the maximum flow rate.

Guidelines differ on how much safety margins to consider for the head and flow. Safety margins as low as 5% or 7% or as high as 20% have been recommended in different textbooks and references. Applying a safety factor on the head also provides reserve capacity on flow and vice versa.

Operational and Maintenance Considerations

Many pumping applications expose pumps to harsh conditions, including extreme temperatures, corrosion, abrasion and other operational challenges. These factors often lead to pump problems, particularly failures such as bearing issues and seal breakdowns. When dealing with extreme temperatures, every aspect of pump design and construction must be tailored to the specific application. This includes carefully selecting appropriate materials, choosing the right type of seals and ensuring all design details can withstand the demanding temperature conditions.

When selecting and sizing pumps and their associated systems, it's crucial to consider the total cost of ownership. This encompasses not only the initial purchase and installation expenses but also the operational costs over the pump's lifespan. The goal is to minimize the overall expenditure, including both upfront and long-term costs.

To achieve this, several operational factors must be taken into account:

System efficiency: Minimize overall friction to reduce energy consumption.
Reliability: Assess and mitigate potential failure points to prevent unscheduled shutdowns.
Energy costs: Factor in electricity or other energy expenses, as these often constitute a significant portion of operational costs.
Maintenance: Consider routine and potential repair costs.
Downtime impact: Estimate the financial impact of unexpected shutdowns on production or revenue.

By carefully evaluating these aspects, organizations can make informed decisions that balance initial investments with long-term operational efficiency and reliability. This approach helps optimize the total cost of ownership while ensuring the pump system meets performance requirements.

Variable Speed Drive Pumps: Adapting to Changing Demands

Many pumps in modern applications are variable speed drive (VSD) driven centrifugal pumps and working in a relatively wide operating speed range. They should be able to operate in a wide range of conditions.

The most common configuration for VSD pumps is “1+1” where one pump is operating and another is on standby. This is the most preferred and most reliable configuration. However, there are cases where multiple pumps, often in “n+1” or “n+2” configurations are applicable. Particular attention is needed when multiple pumps operate in parallel. Initially, operators should start one pump at the minimum VSD speed. They will then increase the pump speed with an additional rise of demand until the pump is at the maximum speed. A further rise in demand will initiate the start of a second pump and then, in turn, a third pump should demand rise with two pumps at full speed. The same can be continued for three or four pumps. The pump stop will follow a similar sequence based on falling of demand.

Horizontal vs. Vertical Pumps: Choosing the Right Configuration

Chemical processing plants commonly use horizontal pumps. However, some applications require large pumps in limited spaces, necessitating vertical pump installations. Engineers typically prefer horizontal pumps unless suction conditions or space/budget constraints dictate otherwise. Vertical pumps provide compact and cost-effective solutions for specific applications.

Proper pump selection and sizing should follow established engineering practices, standards and codes. This approach maximizes the pump's operating service life while minimizing issues from hydraulic loads, vibration, flow separation and other operational problems.

Critical Performance Parameters: From Shut Off to Alignment

The shut-off head and the rise from the rated/operating point to the shut-off head are important. These are critical parameters in certain applications. The shut-off head is the total head that the pump can deliver at zero flow. In certain applications, the pump discharge line may have to run at a much higher elevation than the final discharge point. The liquid should first reach the higher elevation in the system. Flow will not occur if the shut-off head is smaller than the static head corresponding to the high point. During start-up and checkout of the pump, a quick way to determine if the pump has the potential capacity to deliver the head and flow required is to measure the shut-off head. Operators can compare this measured to the theoretical shut-off head predicted by the supplied performance curve of the pump.

The alignment of the pump and its driver is a key requirement for many pumps. As a rough indication, the pump and the driver, usually an electric motor, should be laser aligned to a tolerance of ±0.05mm, or sometimes even better.

Many standby pumps may have been left idle for months or even years, which can lead to startup failure when they’re needed. For this reason, standby pumps should be included in pump maintenance plans, including the cascading pump operation sequence for the pump set or pumping system. Keeping standby pumps idle for long periods is not advisable.

Suction and Priming: Ensuring Smooth Pump Start-up

Since priming can be a major issue for some pumps they should operate under a positive suction head. Each pump should preferably have an individual suction line (suction piping). Suction source and suction piping should be such as to avoid turbulence near the suction and to prevent vortex formation.

Many pumps have been used in batch-type or intermediate operation. The theoretical fill time and minimum pump cycle time should be considered in the sizing of suction source (storage units, tanks, etc.) that feed a pump. For many pumps, suction source lacks sufficient sizing, typically smaller than needed. This has been a widespread problem that can lead to a pump starting and stopping many times per hour. As a very rough indication, the effective volume between pump start and stop should be provided for no more than three pump starts per hour. Choosing pump systems and associated controls capable of functioning across varying delivery rates is crucial.

The pump capacity should be based on the peak flow and should be adequate to maintain the liquid velocity within minimum and maximum limits to avoid operational problems, such as high head loss. As very rough indications, a minimum velocity of 0.7 m/s (usually suction) and maximum velocity of 2.7 m/s (often discharge) is recommended.

Suction Piping: Design Principles for Reliable Operation

The function of suction piping is to supply an evenly distributed flow of liquid to the pump suction nozzle, with sufficient pressure to the pump to avoid excessive turbulence or cavitation. Suction problems can result in cavitation, poor pump performance, low bearing life, mechanical seal failures, high vibration and many more. In extreme cases, vibration produced due to suction problems can cause fatigue failure of pump parts, piping and appurtenances. The cavitation and gas lock, or air lock, can lead to many pump problems and failures. Adequately size suction piping and properly configure it to avoid cavitation and gas/air entrainment.

For suction piping, limit the liquid velocity to values below those where turbulences would occur or bubbles would rise through the liquid.

Also, maintain a minimum velocity to prevent solids from settling if the liquid contains any solid particles or contaminants. As a rough guide, aim for suction piping velocities between 0.7 and 1.3 m/s. Size the suction piping at least one or two sizes larger than the pump's suction flange for many pumps. Lay out the piping correctly to eliminate gas or air pockets, gas-lock and other adverse effects.

The suction piping should be as short and simple as possible to prevent the accumulation of gas from the liquid being pumped. Arrange the suction piping without any bends. If bends are necessary, try to minimize them, and use long-radius bends or long-radius elbows. An important factor for suction piping is to reduce turbulence at the suction particularly immediately to the pump suction nozzle.

Keep the amount of turbulence and entrained gas at a minimum. The liquid flow is further complicated when elbows or tees are located adjacent to the pump suction nozzle (2). In this scenario, uneven flow patterns or vapor separation keeps the liquid from evenly filling the impeller leading to operational problems and reliability issues (3). A straight length of pipe is needed in front of (any centrifugal pump. As a rough indication, install a straight length of pipe equal to six times the pump inlet size (“6×D”). Some specifications or manufacturers recommend “8×D” or even “10×D” for this straight length.

Case Study: Configuring a Flexible Pumping Unit

At one chemical plant, a unit pumps liquid feed from the unloading section to the main processing unit. The facility was designed with the goal of 2,000 m³/h of flow of liquid feed in the first year of operation.

After the first year, other production units would come online and the flow would increase to 7,500 m³/h. In the first year, or first development stage, the operating capacity and head were 2,000 m³/h and 27 m, respectively. In the second year and onward (second development stage), the operating capacity and head were 7,500 m³/h and 60 m, respectively (4).

The plant wanted to use the same pump model for all development stages. The plant also wanted to keep pumps of first development stage running while developing the second stage. Because of the difference in head between the first development stage and the second development stage, the only way to use the same pump model for both stages is to use a variable speed drive (VSD).

As such, the same pump model would work at the lowest speed to provide the lower head (27 m) for the first development stage and at the highest speed to provide the higher head (60 m) for the second development stage. The first group of pumps were two pumps the plant operated as “1+1” arrangement (1 operating + 1 stand-by) during the first year.

The pump works at the lowest speed to provide capacity and head of 2,000 m³/h and 27 m, respectively. For the second development stage, the plant needed five pumps, two existing VSD-driven pumps and three new pumps. These should operate in a “3+2” arrangement (3 operating + 2 stand by). Each pump provides capacity and head of 2,500 m³/h and 60 m, respectively. In this way, two VSD-driven pumps are installed for the first year of operation. Each pump has its own suction line. Proper valves and flanges are provided at the discharge header piping in a way that additional three pumps can be installed later without any interruption of the operation of existing pumps.

Resources