Replacing Centrifugal Water Pumps for High Pressure Process Applications

Introduction

Matching a pump system to a particular application can be a difficult task, especially in today’s energy-conscious society. The increasing use of variable speed drives with centrifugal pumps is a testament to the industry’s desire to improve energy efficiency.  However, the fact remains that centrifugal pumps employed in various processing industries are not always the most appropriate option to select when compared to positive displacement pumps. They are especially inefficient at producing higher pressures where multiple stage pumps or specialty centrifugals such as pitot tube pumps are commonly employed.

One area where high pressure process pumps play an integral role is in the paper industry, where high pressure (400 to 5,000 psi) water jets are used in a variety of continuous-duty cleaning applications. Applications include the cleaning of screens, felts, fabrics, and suction rolls used on the paper machine, as well as the cleaning of filters and fiber recovery systems used to support the process.

Most all of these applications involve pressures and flow rates that are varied to match changing process conditions. While centrifugal pumps are generally viewed as advantageous for pressure and flow control, a large energy cost penalty is paid due to inherent inefficiencies compared to positive displacement pumps. A technology called the UNiGY PhD™ (Pump-modeling hydraulic Drive) technology developed by Kadant AES, a global provider of fluid process solutions to the pulp and paper industry, successfully applies variable speed drive technology to positive displacement pumps.

Significantly different than a Variable Frequency Drive (VFD), UNiGY PhD technology allows coordinated control of both flow and pressure from a positive displacement pump, simplifying controls design and greatly improving energy efficiency. While developed for the demanding duty of pulp and paper making, UNiGY technology has proven equally valuable to other industrial processes.

While the following case study involves a cleaning system used to maintain a lime kiln mud filter used in the kraft pulping process, the concepts involved are easily applied to many other industrial process pumping applications.

The Kraft Process

In paper making, wood is processed to make the pulp from which paper is formed. One method of wood processing, known as the kraft pulping process (also known as the sulfate process,) uses chemicals that, at elevated temperature, break the bonds linking lignin to cellulose, thus liberating the cellulose fibers. Originally developed in the late 19^th century, the modern kraft pulping process is mainly closed-cycle with respect to the inorganic chemicals used to remove the lignin from the wood fiber. The process involves the use of quicklime (calcium oxide), which is itself regenerated by filtering the lime mud from the “white liquor,” using large vacuum drum filters. The thickened lime mud is then calcined in a continuous process lime kiln.

A lime mud filter employs a “precoat” layer of lime mud deposited on a large drum as the filter medium upon which a thickened layer of mud develops. The thickened layer (75-85% solids) is continuously sliced from the precoat layer as the drum revolves, allowing a new thickened layer to develop upon the precoat layer. Over time, the precoat layer becomes more and more compacted and clogged with contaminants, thereby reducing the vacuum drum filter’s capacity. Over time the pores in the drum itself also become plugged which, in turn, reduces filter capacity.

During the 1990s, a better method of lime mud filter operation was developed. Rather than remove the precoat layer entirely on a periodic basis, it was found that the precoat could be renewed continuously in very narrow strips using jets of water from 0.04 in. (1 mm) nozzles sprayed at between 50 and 200 psi (3.4-13.6 bar). Typically the nozzles are mounted on a spray bar, spaced on 12 in. centers across the face of the filter. The ideal precoat renewal system uses permeability of the mud cake as a method of optimizing spray bar pressures through the pressure ranges indicated, as well as determining the need for a periodic higher pressure drum cleaning cycle to maintain constant throughput for optimum kiln performance. During the cleaning cycle, water pressure is increased up to 500 to 1,000 psi (34-68 bar) to remove scale build-up on the drum itself.

How It Works

The wide range of pressures required for precoat renewal and drum cleaning make pump selection a challenge. Initially, systems supplied by Kadant used either multistage centrifugal or Roto-Jet™ (pitot tube) pumps. The table below shows how a UNiGY PhD system is more efficient than a centrifugal pump system, requiring much less power in each mode of operation while extending the range of the drum cleaning system to achieve higher pressures when required.

UNiGY shows drastic efficiency improvements over centrifugal pump systems.

Although the UNiGY PhD has been applied successfully to a wide variety of positive displacement process pumps, for the precoat renewal and cleaning application Kadant engineers selected a Wanner Hydra-Cell™ diaphragm pump as a best match to the range of desired pressures and flows. The UNiGY PhD system uses a proprietary firmware algorithm to precisely monitor and control the positive displacement pump. This algorithm produces a performance model of the pump and the motor. The model dictates how the pump motor should operate in order to properly and efficiently deliver the requested pressures and resultant flows. The UNiGY PhD system increases efficiency and decreases noise levels by applying motor torque required to achieve the desired pressure while simultaneously controlling pump speed (rpm) to exactly match the resultant flow rate. No energy is wasted by reducing pressure across a pressure control valve or by waste-gating excess flow.

As an added benefit, the system monitors all aspects of pump operation in order to warn the operator before a problem escalates. Since flow is precisely monitored, the technology knows if there is a hose leak or a plugged filter. The system is sensitive enough to alert the operator to a single plugged nozzle on the spray bar, an important feature in this application as the spray bar is hidden from operator view.

UNiGY Technology and Its Advantages

Integrating a UNiGY PhD system with a positive displacement pump uses up to 80% less energy than centrifugal equipment, and has proven significantly less expensive to run due to lower maintenance costs as a function of greater reliability. The system is also much quieter because it operates the pump at a lower speed, while maintaining pressure. Being able to select a pressure or water flow rate within a given range allows the user to optimize the precoat renewal process.

UNiGY technology has several advantages over a positive displacement pump with a variable frequency drive system (VFD). A typical VFD provides only speed control. UNiGY integrates speed control with torque control. UNiGY also allows the pump to be operated at lower speeds while maintaining pressure, which improves overall efficiency, reduces noise levels, and increases pump life.

An increase in overall performance, efficiency, greater reliability, and greater precision with a reduction of noise levels and maintenance costs make UNiGY a good choice for high pressure applications. The system has proved effective in over 20 separate lime mud renewal systems installations at various paper mills such as the Simpson Paper mill in Tacoma, Washington