Industrial Utility Efficiency

Optimizing Five Liquid Ring Vacuum Pumps on a Paper Machine


Industrial process operating loads and optimal set points are not usually accurately known at the time of design, so often there is significant mismatch between equipment and the process it serves. To overcome this uncertainty, designers typically oversize equipment. Over time, process changes and equipment efficiencies decline, so equipment might be operating less efficiently than at start-up. Or, equipment can be undersized, thereby hampering the entire system and causing other inefficiencies to compensate. For instance, too much steam usage in the dryer section of a paper machine can occur because of inadequate vacuum at the wet end.

Proper tuning and commissioning rarely happens, so it is not known if the system is operating per the intent of the original design. Typically, there is insufficient monitoring data to even know if the system is still operating at its commissioned level. Finally, system set points usually change over time, and the vacuum system design is usually not set up properly to be able to be adjusted easily and efficiently. 

For all these reasons, some vacuum systems need a complete retrofit to meet current and future standards and process requirements. However, most vacuum systems can be improved quite a bit just by being “re-commissioned.” The easiest re-commissioning is to tweak the vacuum pump speeds (assuming they are belt-driven). As an example, a paper machine vacuum system will be used to illustrate how constant flow vacuum systems work. The example also illustrates one simple way to optimize a constant flow vacuum system—speed adjustment. Any constant flow process can do the same.

The principles I will use in this article are summarized as follows:

  1. The “system curve” needs to be known.
  2. The vacuum “pump curve” needs to be known.
  3. The system curve and pump curve cross at the “operating point,” and should be as close as possible to the best operating point for the system and for the pump. 

 

Principle 1: The System Curve Needs to be Known

A “system curve” is the relationship between vacuum (pressure differential, really) and flow. It tells you how the system “behaves.” How much flow do you need to “feed” it to keep it satisfied, at every potential desired operating point? It’s a bit tricky in a vacuum system, especially if the reader is familiar with the standard liquid system curve (pressure drop is proportional to flow squared). With gas flow at low pressures, pressure differential is proportional to the square of mass flow, and inversely proportional to density—and density changes a lot. So how do you find the system curve in your real process?

First, you need to make some measurements. Trend logging is always better, but if all you have are spot measurements, it can still work. Assuming there is only one operating point for a constant flow system (or subsystem), you only need to measure at that one point. The following measurements are needed:

  • Ambient pressure
  • Vacuum, at the pump inlet, after any throttling valves
  • Vacuum, at the system “exit” point, but before any throttling valves
  • Flow or flow proxy

Ideally, you need to measure with an orifice plate at the inlet or discharge of the vacuum pump, wherever it is practical to have a meter run. Some vacuum systems have these flanges and taps already installed, because the system was flow tested at initial commissioning. The rigorous approach is to use the ASME 9 or 10 procedure, depending on whether it is positive displacement or centrifugal, and the ASME 19.5 method for flow measurement (See References 1 through 3). Or, flow proxies can be made. Your vacuum pump vendor or an outside consultant should know how to do that. A few alternate methods include:

  • Use pump curve, Amps and vacuum. This is the least desirable, since you should always question the validity of any curve.
  • Use a straight run of pipe and a differential pressure (DP). This is essentially a crude flow meter. You really need one DP transmitter, and then use the piping friction tables to estimate flow.
  • Use process instrumentation or test data, correlated with vacuum.

 

Analysis

Convert flow to icfm at the system outlet (before the regulating valve, if there is one).

Calculate alternate flows for other vacuum levels, in 1”Hg increments.

See Table 1 and Figure 1 for an example system curve and estimate of optimal power at each system curve point.

 

systemcalculations

Table 1: Example System Calculations

Assumptions:

  1. System equivalent orifice is not changed
  2. DP is proportional to Qm^2 / density
  3. Qm is proportional to (density x DP)^0.5
  4. Power is proportional to icfm and vacuum
  5. Pamb 29.92 "Hg

System Curve

Figure 1: Example Vacuum System Curve

 

Principle 2: The Vacuum Pump Curve Needs to be Known

Positive displacement vacuum pump curves all tend to look similar, pretty flat flow for all vacuum levels, except at the extremities. Centrifugal exhauster curves look more like a centrifugal pump curve, with flow reducing as head (vacuum) increases, and vice versa. This example is for pulp and paper dewatering, so we are using a curve of a liquid ring vacuum pump.

In reality, no pump matches the curve exactly. If the above flow measurement method can be accomplished for the system curve, you can generate the vacuum pump curve with no trouble, as follows:

  • Install data logging for pump inlet vacuum (not system vacuum) and Amps (or power).
  • If flow can be trend logged, great. Otherwise, make a spot calculation of flow at each operating point.
  • Vary flow lower by throttling the vacuum pump intake/isolation valve.
  • Correlate flow with vacuum and power with vacuum.

 

Principle 3: Optimize Vacuum Pump Performance for an Alternate Speed

If the vacuum level is too high, the pump is “pulling too hard” on the system. If you reduce the speed, the vacuum will drop along the system curve, as described above. Power will also drop.  Savings can be significant. I will explain that for one of the five pumps in the example project. It can be done as follows:

  • Adjust vacuum pump speed to move the operating point as close as possible to the needed vacuum level. For belt-driven vacuum pumps, this is simple. For direct drive, it would require a VFD.
  • Stay within the pump curve speed range limits. 

 

Example Paper Machine Project with Five Vacuum Pumps

 Paper machines use vacuum in the forming, press, and drying sections, as can be seen in Figure 2. In forming, the largest flow requirement exists. Vacuum and gravity pull a large amount of water out from liquid stock (starting at only 4 percent solids in the “headbox”). This is known as sheet formation, where the fibers start to spread and consolidate into a thin mat. It looks like a white fleece in this section. The web of wet paper is then lifted from the wire mesh and squeezed between a series of presses where its water content is lowered to about 50 percent by squeezing between rollers. It then passes around a series of cast-iron cylinders in the drying section, heated to temperatures in excess of 200°F, where drying takes place. Here, the water content is lowered to between 5 and 8 percent—its final level.

 

papermachine

Figure 2: Typical Paper Machine

 

The vacuum process evaluated in this article is in the press section. In this part of the paper machine, air is sucked through the web in long slots on the top of a box called an “Uhle box” (Refer to Figure 3). The press section has several Uhle boxes, each designed with exactly the right slot geometry and vacuum level to remove water step by step.

 

Paper Machine

Figure 3: Typical Uhle Box of a Paper Machine (Vacuum Connection on Back Side)

 

The example project’s vacuum system served the press section of a paper machine, and it is shown in Figure 4. It uses positive displacement vacuum pumps called liquid ring vacuum pumps, that use a “liquid ring” for the cylinder wall and a rotor that is immersed in the ring, as shown in Figure 5. They can handle high condensable loads, and they are common in wet applications like pulp and paper. Centrifugal exhausters are starting to make inroads into pulp and paper, and have efficient turn down, which is valuable if a paper machine changes products and requires multiple vacuum set points. In typical paper machines that run the same product all the time, liquid ring pumps are usually the most economic choice overall. They are fairly efficient and highly reliable. However, they do have limited speed ranges. 

All of the vacuum pumps in this example system operate at different vacuum levels. The exact levels required for each were not known by the customer at the time of the assessment. It was proposed that 10”Hg should be a starting point, since several of the Uhle boxes were operating at 10”Hg.

flowcalculations

Figure 4: Project System Diagram

 

Speed Adjustments Optimize Vacuum System

The remainder of this article will show the savings accrued by adjusting the speed of VP1 so that it could operate at 10”Hg. Please refer to the vacuum pump curve, superimposed with the system curve, in Figure 6. The curve is not validated by testing. It is just the manufacturer’s data. The operating point, vacuum and power were measured. Flow was estimated from the curve.

To hit the lower vacuum level, 10” HgV, on the system curve, the pump would have had to operate at a lower speed than the minimum of 300 rpm. Thus, the unit must operate at 300 rpm.  Figure 6 shows the dramatic power reduction that can be gained by merely changing the sheaves on the belt drive so the that pump operating speed would be reduced from 360 to 300 rpm.  Power reduction would be about 37 percent—not as much as the ideal 50 percent (if the pump speed could be further reduced).

flowvsvacuum

powervsvacuum

Figure 6: Vacuum Pump #1 Performance Curves

 

If a constant-flow vacuum pump is operating at a higher vacuum than needed, simply reducing the speed can garner significant savings. However, the system curve and pump curve need to be known, so the pump operates at the correct operating point.

 

For more information, contact Tim Dugan, tel: (503) 520-0700, email: Tim.Dugan@comp-eng.com, or visit www.comp-eng.com.

To read more about Vacuum Pump System Assessments, please visit www.blowervacuumbestpractices.com/system-assessments.

 

References

  1. Displacement Compressors, Vacuum Pumps And Blowers, ASME PTC 9 – 1970.
  2. Performance Test Code on Compressors and Exhausters PTC 10 – 1997
  3. Flow Measurement, ASME PTC 19.5 - 2004
  4. Energy Efficiency Improvement and Cost Saving Opportunities for the Pulp and Paper Industry, An ENERGY STAR® Guide for Energy and Plant Managers. Ernest Orlando Lawrence Berkeley National Laboratory, October 2009, Chapter 17, Paper machine vacuum system optimization, pp 104