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How to Avoid Problems With Your Hydronic System Pumps
How to Avoid Problems With Your Hydronic System Pumps
Some hydronic systems continuously have trouble. The owner of such a troubled system is paying service bills to repair or replace various components that are constantly failing. Service technicias such as unheatable circuits, noise, air binding, excessive component failure, especially pumps, etc., needs to be analyzed to discover the reasons for the constant trons find the failed component, replace it, and tell the owner the system has been “fixed.” Any system that has continuous problemuble. Properly designed, installed, and started hydronic systems will be trouble-free for many years.
Hydronic engineers who have plans and specifications usually design large hydronic systems. As long as the installing contractor follows the plan and spec, no system problems should be encountered. Smaller systems, residential and commercial systems, are usually “designed” by the installing contractor. These systems can exhibit ongoing problems, and instead of just replacing parts, need an analysis to discover the real problems.
Many mistakes are made locating circulating pumps in relation to the expansion tank. When pumps were first used, they were always located on the return piping, pumping into the boiler. This was where the water was coolest, having circulated through the system and given up its heat. Manufacturing tolerances could not be as closely held as they are today, so where the water was coolest was the norm for locating circulating pumps. As we will see, this “standard” is obsolete and not necessarily the best location for the booster pump. Manufacturing processes have improved so that the pump can be located in the boiler discharge water with no adverse effect on the pump. The pump location is determined by where the expansion tank is connected to the system.
When the pump is off, the only pressure existing is static pressure (see Info-Tec 26, Hot Water Heating Systems). Starting the pump will change the system pressure to a new set of conditions. The pump head will appear across the pump. The pressure at the pump discharge will be higher than the pressure at the pump suction by an amount equal to the pump head. Pressure drop (DP) will gradually decrease from the discharge to the pump suction.
By specifying a point of no pressure change, system pressures can be governed when the pump is on. The point of no pressure change is where the expansion tank connects into the system. This is because the air in the compression tank must follow gas laws: a change in air pressure must be followed by a change in air volume. A change in air volume results in a change in water volume in the tank. A change in water volume in the tank must cause a change of water volume in the system. Pump operation cannot increase or decrease system water volume, since water is incompressible. Therefore, pump operation cannot change tank pressure. Since tank pressure cannot change because of pump operation, the junction of the tank with the system must be a point of no pressure change.
Based on this fact, if the compression tank is placed at the suction side of the pump, the pump suction pressure will not change, regardless of whether the pump is on or off. Since the pump suction cannot change, the pump discharge must change when the pump is on. All the pump head must show up as a positive increase at the pump discharge. The pressure increase will decrease through the system to the original static pressure at the pump suction. (This is called hydronic gradient.) This is graphically represented in Figure 1. Note the line representing the pump head or hydraulic gradient. It is above the original status pressure line throughout most of the system.
Because the suction pressure is unchanged from static pressure due to pump operation, this is the best location for the boiler (See Figure 2).
If the compression tank is located at the pump discharge side, with the pump pumping into the tank and boiler, all system pressure changes due to the pump running will be subtracted from the original static pressure. Since the pump discharge pressure cannot change, the suction pressure must change. (See Figure 3.) The suction pressure will show a drop equal to the full pump head. Boiling or cavitation may result. The pressure decrease at high points of the system may be enough to cause a vacuum, sucking air into the system through vents. Air-bound circuits may result. Unstable, unbalanced water flows could result. Noisy, cavitating pumps will soon fail. The boiler may “pound” each time the pump runs.
For systems that exhibit these problems, and where the pump is discharging into the boiler and compression tank, three fixes are possible:
1. Raise the static pressure high enough to prevent air suction and boiling. This may require resizing the compression tank.
2. Turn the pump around. Pump away from the boiler and tank. Often it is not possible to reverse the flow due to monoflo tees, flow valves, etc.
3. Move the pump to the other side of the boiler and compression tank. Pump away from the boiler and tank.
One small, low pump head systems, such as those that use a series 100 or SLC Bell & Gossett pump, it may not be necessary to pump away from the boiler and tank because the pump energy is not enough to affect system pressures very much. It certainly does no harm to put a system together correctly and prevent problems. In general, systems that require pumps with 1/3 H.P. motors or more should definitely be installed pumping away from the boiler and compression tank.
Since the circulating pump is the major moving part of a forced hot water heating system, not only is its location important, but also proper maintenance is critical to good system performance.
All booster pumps are centrifugal pumps. They use centrifugal force to move liquid. The impeller is the key part. Liquid entering the eye of a rotating impeller is thrown out to the edge with considerable force. The direction of rotation of the impeller is important. The impeller vanes must “slap” the water, not “dig in.” With new single-phase pumps, this is usually not a problem, but three-phase motors are field-wired and can be wired to rotate in either direction. Unfortunately, an impeller spinning in the wrong direction will circulate some water, but capacity (GPM) will be very low and the pump will be noisy.
Motor load or amp draw is dependant on GPM pumping rate. The pump will find the point on its curve where the system DP will just equal the pump’s ability to produce the head necessary at that flow. Figure 4 illustrates a typical pump curve. Flow in GPM is plotted against DP in feet. Motor load is shown to illustrate what happens as GPM increases.
Booster pumps require a flooded suction; that is, a constant supply of clean, bubble-free liquid entering the eye of the impeller, to operate. Often, a booster pump is oversized by the contractor to “be sure” it will pump the required GPM. An oversized pump will result in a noisy system. Therefore, if a booster pump has to be throttled for any reason, the throttling valve should be on the discharge side of the pump. This maintains a flooded suction and prevents cavitation, which will quickly ruin an impeller.
Whenever a pump motor draws excessive amperage and the voltage is within normal limits, gauge readings should be taken. If the readings indicate the pump is oversized and pumping too much water, the discharge can be throttled. In order to check the performance of a pump installed in a system, the differential pressure between the suction and discharge openings of the pump needs to be determined. Once this is found, by referring to the performance curve for the pump, GPM will be known. Figure 4 illustrates the relationship between DP and GPM.
Some pumps will have taps provided to install gauges. If no taps are provided, the pump body can be drilled and tapped or gauge ports installed in the immediate adjacent piping. Make sure both gauges are zeroed and accurate. Subtract the suction reading from the discharge reading. The answer is the head. Pump curves show DP in feet of head. To convert the gauge readings in psi to feet of head, multiply psi by 2.3. As an example: Figure 4 is the pump curve for a pump that shows a 2psig differential when running. Multiplying 2 psig x 2.3 equals 4.6 feet of head. Enter the pump curve chart at 4.6 DP and draw a line to intersect the pump curve. Drop a line from this intersection down to the GPM line and read 18 GPM.
In theory, an oversized pump could be throttled to very low flow, even no flow, with no damaging results. In actual practice, this is not true. While the motor is unloaded at low flows, the energy of the spinning impeller has to “go” somewhere, and that somewhere will be into heat. This heat of friction can cause boiling in the pump impeller housing, ruining the impeller and/or pump seals. If a pump is so oversized that its flow must be throttled more than 50%, it is better to replace the pump with a properly sized one, rather than only throttle it.
While most operating system pump problems are due to oversized pumps, undersized pump problems should be addressed too. Most undersized pump problems are due to an addition being made to a system and not recalculating the new parameters for the system. An undersized pump installed on a new system is usually discovered right away and fixed. It is when existing systems are added that the pump is forgotten about and circulation problems develop. Any system that experiences heating problems after additional radiation is added is suspect to an undersized pump problem.
A large system temperature drop evidences insufficient circulation. If there is more than one circuit, the short circuits may heat well, but the longer ones don’t. If rebalancing the system cannot correct the insufficient heating problem, suspect an undersized pump. Using gauges as before, one can check the pump.
There are some rules of thumb that can help determine pump capacity:
Pump capacity can be determined by dividing the calculated BTU/Hr. heat loss of a building by the BTU/Hr. capacity of each gallon per minute circulated. Using the definition of a BTU, if one pound of water drops one degree Fahrenheit as it circulates, then one BTU is given off. A gallon of water weighs 8.3 lbs. Therefore, if a gallon of water drops one degree, it has given up 8.3 BTU’s. If one gallon per minute is circulated for one hour then: 8.3 x 60 = 498 BTU/Hr. Use 500 for easier figuring. The design water temperature drop, usually 20oF, multiplied by 500, equals 10,000 BTU/Hr. per gallon circulated. If a building’s heat loss was 200,000 BTU/Hr., the pump has to pump 20 GPM. (The actual operating temperature drop will probably be a lot less than the design temperature drop. This will not change the output of radiators to any considerable degree.)
Most insufficient circulation problem complaints on systems that have not been added to, are traceable to air binding. No boiler air-trol system is 100% efficient. Some air is always entrained in the water and circulates with the water. IF the system was not properly started, large amounts of air are still in circulation in the system. Eventually, the air will rise to the high points of a system where it will act as a break in the system. A circulating pump cannot push air down a vertical pipe.
Each system high point requires a vent in order to purge air out of the system. A gurgling sound at the return side of a radiator is evidence of a partially air-bound radiator. If a system continues to have air-binding problems, the cause of excess air getting into the system must be found. Besides causing no heat or insufficient heat problems, excess air can ruin the components of a system.
1. Check for leaks; especially pump seals.
2. Is the line to the tank fitting properly – sized and pitched?
3. There should be no valves in the horizontal line to the tank or street ells in the boiler or tank fitting openings.
4. The dip tube of the boiler fitting should be pushed into the boiler as far as it can go.
5. If automatic air vents are used in the system, change to manual vents.
6. And finally, do a proper start-up as previously described in Info-Tec 26 (Hot Water Heating Systems).
Figure 5 shows a typical installation and points out the items enumerated above.
If a system was properly started, installed, and thoroughly checked, and yet air-binding is still a problem, a check for gas generation should be made. The various materials used in installation, such as solder fluxes, cutting oils, pipe compounds, etc., when heated, can cause a chemical reaction and produce a combustible gas. This gas is being generated constantly, and no air control system can cope with it. The system needs to be cleaned. All systems should be cleaned after installation and before startup, but seldom are.
Cleaning can be done using trisodium phosphate, a caustic soda, or a TSP substitute. A ratio of one lb. of TSP to 50 gallons of water in the system is recommended. The TSP should be dissolved in hot water and then added to the system in liquid form in any convenient way. Let this solution circulate for at least several hours. The system should operate at normal heating temperatures during this time. Do not circulate this solution for more than 10 to 12 hours. After circulating, drain completely and refill the system with untreated, clean, fresh water. (If a glycol system, glycol can now be mixed and filled.) Circulate the filled system, cold, for 10 to 15 minutes. Now, check the system water with PH indicating papers. The system should show a PH between 7 and 9. If low (acid) add some cleaning solution to bring up the PH, but do not exceed 8. High PH (alkaline) should be avoided.
Once the system is cleaned and the PH level is good, the system should be properly started.
Properly installed hydronic systems are inherently quiet. Any noise loud enough to cause a complaint from the building occupants should be investigated. If the noise exists only when the pump is running, do not immediately assume the pump is at fault. In many cases, it is not the pump, but an installation problem.
Expansion and contraction of the piping will be noisy, unless proper care was taken to absorb the expansion of the piping system. A 10’ piece of 3/4” copper tube will expand 7/16 of an inch at a 100oF temperature rise! This expansion must be allowed for or a lot of noise will result, even damage the piping system and adjacent structural components.
As has been noted, entrained air can cause circulation noises, and an oversized pump can cause circulation noises.
Any equipment with moving parts will generate some noise and vibration. Where piping noise is induced by pump vibration, the pump should be checked. On smaller boosters with ring-mounted motors, misalignment because of a bent motor bracket, caused by dropping or stepping on the pump, will cause vibration. Oil soaked motor mountings will sage and cause misalignment. Over-oiling of the booster motors has caused more failures than under-oiling. Misalignment will cause excess wear and frequent failure of couplers. Couplers and motor mounts should be changed at the same time. In-line pumps should be as close to the boiler as possible in order to avoid putting the strain of the pump’s weight on the piping.
Base mounted pumps should be well mounted to a heavy foundation, isolated from the floor slab. No piping weight should be imposed on the pump body. Flexible connectors between the pump and piping are an excellent way to prevent vibration transmission. For good isolation, piping should be anchored on the system side of a pump.
Hangers that place strain on a system piping can create noise. Check all hangers. Simply loosening, moving, or replacing a hanger has solved many noise complaints. Risers should never be in contact with a building’s structure.
Frequent seal failures on mechanical seal pumps are usually due to water conditions. All seals leak a small amount of water. It helps to lubricate the seal faces. In fact, on large pumps with packed seals, the packing nut is adjusted to control a specified leak rate. System sealers plug leaks by solidifying when in contact with air. Sealers will cause rapid failure of the seal faces. If a sealer is ever used in a system, it should be drained out as soon as the leaks are sealed and the system refilled and started again. Many additives, such as corrosion inhibitors, when used in excessive amounts, can also cause seal damage. A pump should never be run dry. The pumped fluid carries away frictional heat generated by the seal, besides helping to lubricate the seal faces.
Booster pumps are designed for closed systems. They cannot handle large amounts of fresh water. They will experience seal failure, pitting of the pump body, and impeller destruction. Pumps used for potable water circuits are all brass construction, for the reason just stated, and even then do not have the usual long life of a closed system pump.