Creating power through coal requires all sorts of machinery, and if one piece of the system fails, problems arise. Coal pulverizers require pumps to circulate oil to maintain function, making them a critical part of the operation. Recently, we encountered a very large company that used sub-optimal pumps. They were dealing with constant leaks and poor service life, so we suggested upgrading to a better pump.
This client has 14 pumps--seven in use, and seven on standby, totaling two for each pulverizer--so they wanted to do a trial with one pump before committing to such a large order. We suggested they install a Viking KK4124B, a high-level product designed for lots of use. The superior reliability of the Viking pump would not only save the client money over time, it would limit the hazard issues caused by oil leaks. It did not take long for the client to notice huge improvements, and they decided to order 13 more pumps.
In addition to offering this client a much better product compared to their previous pump, we were happy to provide the service required to make the changeover a success. We created a custom base for the Viking pump so that it could slip in and replace the existing pump with minimal difficulty. We want all of our clients to experience the benefits of the products we supply, and do not mind going the extra mile to make the transition as smooth as possible.
When you genuinely believe in the quality of your products, as we do here at Pye-Barker, convincing people of the benefits is a fun and exciting process. We know that once a client sees how much money and hassle better equipment will save them, they will understand our enthusiasm.
Two common descriptions of pump pressure are PSI (pounds per square inch) and TDH (total dynamic head). PSI is usually associated with positive displacement pumps and TDH with centrifugal pumps, but the terms are interchangeable.
PSI is usually stated in gauge pressure as PSIG, the pressure you read on a pressure gauge. Negative pressure is usually stated in inches of mercury vacuum, HgV. These are common values used in America, but pressure can be translated into metric and other terms.
Differential pressure (ΔP or delta P or DP) is the difference in pressure across the pump. Differential pressure is used for pump and motor selection as it is the actual pressure the pump sees. DP is the pressure on the suction side of the pump plus the pressure on the discharge side of the pump in relation to the desired discharge pressure. If 50PSI is the desired discharge pressure and you have +10PSI on the suction side, the pump needs to develop 40PSI for the total discharge pressure to equal 50PSI (+10+40). Likewise, if the suction pressure was negative -5PSI (~10”HgV) such as a suction lift application where the liquid level is below the pump, the pump would have to develop 55PSI (-5+55) to achieve the total discharge pressure of 50PSI.
PSIA is pressure per square inch absolute. This takes into account atmospheric pressure. 0PSIG is equal to 14.7PSIA at sea level or 33.9’ of water column (atmospheric pressure decreases with altitude, for example: 14.2PSIA at 1000 ft. elevation).
TDH or total dynamic head is another common term for describing pressure across a pump. Every 2.31’ of vertical level = 1 PSI for water and liquids with a specific gravity of 1 (or 8.34 pounds per gallon). For liquids heavier than water, the pressure exerted is greater for the same vertical level. For example; a tank with 23.1’ of water level would read 10PSI on a pressure gauge at the bottom of the tank [(23.1ft./2.31ft/psi)X1SG=10psi].
With a liquid with a specific gravity of 1.2 (10 pounds per gallon) at the same 23.1 foot level, the pressure gauge would read 12PSI [(23.1ft./2.31ft/psi)X1.2SG=12psi].
And a pressure gauge would read less for liquids lighter than water.
The pump manufacturer will state the NPSHr, Net Positive Suction Head required, to prevent cavitation or vaporization of the liquid in the pump. The NPSHa, Net Positive Suction Head available, must be greater than that required by the pump.
For a basic understanding of Net Positive Suction Head, you must think in terms of absolute pressure in feet. To determine the NPSHa, the Net Positive Suction Head available at the pump inlet, you first determine the pressure above the liquid. As atmospheric pressure varies with altitude (and weather), the site elevation must be known. For this discussion, we will be at sea level and pumping water.
At zero feet elevation, sea level, the atmospheric pressure is 33.9’ absolute (or 14.7psia). When you are pumping from a tank that is open to atmosphere, you automatically have 33.9’ absolute pressure helping you get liquid to the pump suction.
We will assume our tank has 10 feet of cold water level above the pump. We have a total of 43.9’ of pressure available above the pump (33.9’ atmos. pressure + 10’ liquid level).
That 33.9’ of atmospheric pressure is reduced first by the vapor pressure of the liquid (vapor pressure is the tendency for the liquid to become a vapor).
Using water as a common reference. At 33°F., the vapor pressure of water is basically zero. As the water temperature increases, so does its vapor pressure. At 212°F, the vapor pressure equals 33.9’, the point at which water turns to a vapor.
If we heat our water tank example to 212°F, the atmospheric pressure of 33.9’ minus the vapor pressure of 33.9’ leaves us with only the 10’ of liquid level to supply the pump.
The last part of the equation is deducting the friction loss of the suction piping. This calculation will vary from application to application.
Net Positive Suction Head Available = pressure above the liquid + liquid level above the pump (negative if level is below the pump) – the vapor pressure of liquid at pumping temperature – the friction loss of the suction piping.
Although NPSH calculations are not exact, they but must be considered for proper pump selection.
Pumping abrasive liquids are always a difficult application. The hardness and percentage of solids increase the wear in the pump and piping. Pump speed and pressure directly affect pump wear. Pump speed can be addressed by oversizing the pump to reduce the RPM of the pumping elements. This lessens the abrasion effect and the NPSH required to prevent cavitation. The suction side of the pump should be oversized to ensure a positive flow to the pump. Oversized strainers also reduce suction side pressure drop and protect the pump from foreign objects.
Reducing the discharge pressure is most effective way to increase pump life. Reducing the discharge pressure is entirely in the design of the piping system downstream of the pump. Enlarged piping, flow meters, heat exchangers, etc. all help to reduce liquid velocities and lower the friction losses that the pump has to overcome to get the fluid to the discharge point.
Most pumps that have been in abrasive service for any length of time are worn to the point that the rebuild cost exceeds that of a replacement pump. The pump is required to be a sacrificial component in these services by necessity. After all the piping system improvements have been made, there are several ways to maximize the pump life. We start with the Abrasive Liquid pump with hardened steel gears and the standard Tungsten Carbide Idler Pin and Bushing.
One of the more common applications is metering a filled viscous liquid to maintain a precise flow rate. It is essential that the pump be operated on variable speed drive. The motor must be oversized to allow cooling at low RPMs and sufficient torque at higher speeds. A gear ratio is selected to provide the desired flow rate at a low motor speed, for this example; 33% of rated RPM or 20Hz (the lower the better). As the pump wears, the VFD will increase the pump speed to maintain its flow meter set point. As the motor speed approaches a predetermined speed for the system, for example; 85% of rated RPM or 51Hz; during a shutdown the thrust bearing is adjusted in place, reducing the rotor end clearance to compensate for wear. When the system is re-started, the VFD will operate at a lower speed, perhaps 30-35 Hz, to maintain the flow meter set point. The next time the pump speed may get to 60Hz before the next end clearance adjustment is made. Most modern motors can operate at 88-90Hz without any problems, inverter-duty can go higher. Most VFDs can go to 120Hz with some even higher. Operating above 60Hz varies from a personal preference to plant management establishing limits. So check what is standard or allowed in your facility or inherent in the system design and programming. Precautions are to be considered in motor selection as horsepower drops off proportionately with motor RPM below 60Hz and torque drops off proportionately above 60Hz. The pump manufacturer publishes a maximum pump RPM for a given viscosity to prevent cavitation, not to be exceeded, this can be translated into a maximum operating hertz for a given pump size, to be compensated for based on system design and customer experience. A half worn out pump cost the same to replace as a fully worn out pump.
Some systems require constant recirculation to prevent the solids from settling out. To extend pump life, the recirculation loop should be designed with minimal back pressure and operate at the lowest RPM required to keep the solids in suspension.
In critical applications, or highly abrasive filler applications, or to have the minimal downtime, the interior surfaces of the pump can be hard coated with an abrasion resistant material to increase pump life to its maximum.
Sometimes budget or desperation will lead a customer to purchase used equipment.
However you need to remember that even while well maintained and documented equipment commands a higher price, you still have no guarantee.
Public or online auctions commonly offer low cost used equipment with little or no history.
I have a customer who purchased a used compressor that "looked" good online. But soon after he put this bargain purchase into service he found out that the coolers were plugged. This lead to a high cost major repair before the first service was even due on the unit, and so it turned out this was no bargain.
Down time, labor, and parts added to the bargain purchase price ended up being almost as much as a new compressor. And a new compressor would have come with a warranty!
When it comes to material pumps the motto is the same. Used pumps are just that, used. They typically require a rebuild at minimum and generally are not the right configuration for an application.
Remember the old maxim - "You get what you pay for".
CFM or Cubic Feet Per Minute of air flow is a term that is often confusing in compressed air systems. It is also seen as SCFM, ACFM, and ICFM. So what are the differences?
SCFM or Standard Cubic Feet Per Minute is referenced to cubic foot of air at standard pressure, temperature, and relative humidity. In most cases, SCFM is based on 14.7 PSIA, 68°F, and 36% relative humidity. By these specific parameters, the density of a cubic foot of air is fixed. The mass flow of compressed air is therefore clearly defined.
ACFM or Actual Cubic Feet Per Minute is volume air that is constantly changing due to atmospheric conditions and/or compressed or expanded by air compressors or vacuum pumps. As temperature increases, the volume expands. As pressure increased the volume decreases. And vice-versa.
P1V1=P2V2 Boyle’s Law, with constant temperature, absolute volume changes with pressure
V1/T1=V2/T2 Charles’ Law, with constant pressure, volume changes with temperature.
ICFM or Inlet cubic feet per minute is a unit adopted by equipment manufactures as it relates to a volume air that can be displaced by the equipment. The actual mass of the inlet air varies with atmospheric conditions. The actual discharge volume varies with pressure and temperature.
If the inlet conditions happen to be at 14.7 PSIA, 68°F, and 36% relative humidity (standard conditions), a 1000 ICFM compressor at 100PSIG would produce 1000SCFM at the discharge. But when you convert that to ACFM (14.7psia X 1000CFM)/114.7psia = 128ACFM at the discharge (assuming constant temperature). From atmospheric pressure, you have compressed the air 7.8 times to 100PSIG.
As often misunderstood, this compressor example does not discharge 1000CFM but rather takes it in the inlet.
Some things can be made to work; but just because it works doesn't mean it's the best approach.
I had a customer in Columbus who was metering vegetable oil to spray on baked goods on a conveyor belt. The excess oil would drain to a pan and was then screened before recirculating to the pump.
Some 28 odd years ago, a surplus Viking H32 internal gear pump was installed with a direct drive 1HP 1750RPM motor. A variable speed drive and flow control needle valve were added to control the flow of the oil.
However, the actual flow needed is 1.5-2GPM and the Viking H32 is capable of 16GPM at 1750RPM even though the max speed for the pump is 1150RPM or about 10GPM. Also, the shaft was not being supported by a bearing bracket which is recommended for this pump and the pump had Bronze Bushings which are not suitable for this vegetable oil service.
To further complicate the problem the variable speed drive could not run slow enough to meter the oil without tripping the motor (horsepower is proportional to speed (Hz)) which left the flow control needle valve doing most of the metering work but this would easily clog with crumbs. The needle valve restricted flow causing the pumps 50PSI relief valve to be continuously open and bypassing, and creating backpressure and increasing power requirements limiting the variable speed drives effectiveness.
As a solution we installed a Viking G32 with carbon graphite bushings and a bearing bracket. We used a 3/4HP gear motor with 410RPM out for 2GPM at 60Hz, and utilized their existing variable speed drive.
This completely eliminated the needle valve and now flow control is done solely with the variable speed drive.
Finally we installed an Eaton duplex strainer downstream to better collect the small crumbs.