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This month, we continue with the checklist and discussions for commissioning and troubleshooting domestic hot water systems. We focus on thermal expansion tanks, temperature gauge locations and types, temperature effects on Legionella bacteria and hot water temperature maintenance cables.
Thermal Expansion Tanks
Every hot water system should include a thermal expansion tank or a means to relieve thermal expansion, as required by the model plumbing codes. Verify that a means to relieve thermal expansion pressure is installed and properly sized and located in the cold water line to the water heater(s).
Document the following:
Thermal expansion tank manufacturer;
Thermal expansion tank model number;
Thermal expansion tank serial number;
Thermal expansion tank volume (gallons);
Thermal expansion tank air charge (pressure) (psi)(Thermal expansion tank pressure should be a couple of psi higher than the system pressure.);
Does the air valve (Schrader valve) leak water? Location/room number;
Equipment ID:
• Hot water system served;
• Hot water system design temperature (F) (from plans, specs and schedules);
• Actual system operating temperature.
Temperature Gauges
Temperature gauges should be in the following locations:
• On the cold water pipe feeding the water heater or hot water storage tank before the circulated flow mixes with the cold water.
• On the hot water pipe leaving the water heater. This is to analyze the hot water distribution system temperatures when monitoring and adjusting hot water system temperature controls.
• On the cold water pipe feeding the thermostatic mixing valve (TMV) serving the hot water distribution system.
• On the hot water pipe feeding the TMV serving the hot water distribution system.
• On the hot/tempered water outlet water pipe of the TMV serving the hot water distribution system (to see what temperature it is set at).
• Near the end of the hot water return system to verify that the hot water return temperature is a couple of degrees above 122 F to prevent Legionella bacteria and other microorganisms from growing in the hot water distribution system.
• On the inlet and outlet of pre-heaters or heat exchangers (if any).
• On the inlet and outlet of solar systems.
Temperature gauge notes:
1. Check the hot water return temperature. If it is below 124 F, adjust the hot water distribution temperature (at a master temperature-actuated mixing valve conforming to ASSE 1017 or at the water heater thermostat) so the hot water return temperature is a couple of degrees above the Legionella bacterial growth temperature of 122 F.
This requires all showers and tub-showers to include code-compliant control valves for safe installation; the old, two-handle valves without temperature or pressure compensation and maximum temperature limit-stop adjustments should not be allowed anywhere! The maximum temperature limit stops should be set to a safe temperature for bathing or showering.
The code requires a maximum of 120 F, but I prefer a temperature closer to 110 F; it is a much safer temperature with respect to scalding hazards for a maximum hot water temperature at showers and tub-showers. In some locations, the shower temperature may need to be a couple of degrees hotter to account for ambient temperature conditions in colder climates with glazed tile showers or cast-iron bathtubs.
2. Readjust maximum temperature limit-stops. After any adjustments are made to the water heater thermostat or the set point of the TMV serving the hot water distribution system, the max temperature limit-stops must be readjusted on all showers and tub-showers. Also check the temperatures on other fixtures with code-required maximum temperature limits to ensure the temperatures are safe.
3. Provide documentation to the building owner. If the system temperatures are adjusted and the building owner refuses to readjust the maximum temperature limit-stops or to verify the temperatures are safe, provide documentation to the owner on how it should be done.
Also, provide to the owner or owner’s representatives the manufacturer’s installation and maintenance literature for adjusting the shower or tub-shower valves and temperature-limiting valves. Document the issue in writing, advising them that you have made them aware of this issue.
If you are not getting paid to perform this task, it is the owner’s responsibility to perform the limit-stop adjustments before occupancy, and seasonally check and adjust the maximum temperature limit-stops to account for seasonal changes in the incoming cold water. Put a copy in your file and send one to the building owner, the building manager and the code official who will perform the inspection.
Pressure and Temperature Gauges
Many factors go into the decision of what type of pressure or temperature gauge to specify for an application. Below are some considerations for choosing a gauge, including type, size, location, application, fluid type, temperature, pressure, end connection type. Also, accessories that may be needed, such as pressure gauge snubbers, gauge valves and thermometer wells.
When specifying a pressure gauge, engineers must go through a checklist or similar process to determine which gauge is needed and where it should be in the system. For pressure and temperature gauges, here are a few specification considerations:
• Pressure gauges. There are two basic types of pressure gauges: water-tube manometers (no method to measure or know the actual high pressures inside a closed steam pressure vessel because manometer water tubes would simply blow out at higher pressures) and analog pressure gauges (which use what is called a Bourdon tube, named for the Frenchman Eugene Bourdon).
The manometers were not useful in measuring the actual high pressures inside a closed, steam pressure vessel because the water tubes would simply blow out at higher pressures.
Bourdon conducted experiments and found that, for flattened and bent tubes, as the pressure increased, the tube straightened out and regained its circular cross-section. He ultimately invented a pressure gauge based on the tip deflection of a curved tube with an elliptical coil of tubing. The modern analog pressure gauges are still based upon this Bourdon tube movement phenomenon.
Modern pressure gauges use a controlled thickness and shape of a flat oval tube bent into an arc. It has linkages that move as the pressure increases and the arc tries to straighten out. Knowing the pressures that create a given movement of the Bourdon tube, a series of linkages and gears can be assembled to allow the tube’s movement to cause a dial to rotate on the face of the gauge.
• Digital vs. mechanical. The choice of mechanical or digital pressure gauges is a matter of cost. Digital pressure gauges cost a little more, but they can digitally record and store pressures over time and allow recall of the high and low pressures for system evaluation.
They also can be hard-wired or connected by Bluetooth to the building management computer system to notify the building engineer or others of a pressure or temperature anomaly in the piping system. A digital gauge can be accurate up to ±0.025% of span and are, therefore, more expensive.
Most industrial processes do not require the digital level of accuracy or the number of pressure or temperature recording features. However, with increasing requirements for water management programs and the added requirements for monitoring, recording and documenting of pressures and temperatures in the system to assure conformance with temperatures required to control Legionella bacteria growth, digital pressure and temperature gauges offer a way to monitor the critical control points in a system.
This is outlined in ASHRAE 188, Legionellosis: Risk Management for Building Water Systems, where facility staff need not go around daily and physically read and record temperatures or pressures. Otherwise, if a building does not need a water management program for control of bacteria and microorganisms in the building water systems, a mechanical or analog pressure gauge would work, and temperatures and pressures are monitored or logged manually.
Selection Considerations
There are a couple of things to consider when selecting temperature and pressure gauges:
• Dial face size. Mechanical pressure gauges come in a variety of nominal sizes, pressure and temperature ranges. The type you specify depends on your requirements for readability, space and precision. For pressure gauges, the larger the dial face, the more gradations it will have for more exact readings, and the easier it can be seen from a distance — an important consideration if technicians cannot get close to the gauge.
For temperature gauges, they can be dial-type or liquid-filled glass bulb thermometer-type. Glass bulb thermometers can be difficult to read because you need to be at the right angle. Some applications don’t have room for a large dial-type gauge. This might be a good place for a digital gauge that can, in some models, allow remote monitoring or reading of the temperature or pressure.
The other type of gauges are digital pressure or temperature gauges. They are available in a variety of pressure or temperature ranges and display sizes. The gauge should be selected for the pressure or temperature range it is intended to monitor; the dial face or digital readout should be clearly visible from the mechanical room floor.
If a pipe needing pressure or temperature monitoring is located high in a mechanical room, the dial face and readout should be in larger letters to allow proper reading/monitoring of the gauge from the mechanical room floor.
• Inlet connection size. Another specification factor to keep in mind is the size of the inlet connection of the gauge, which should be coordinated with the tee sizes for the pipe to handle the gauge. Small gauges may only need a 1/4-inch tee, while large gauges may need a 1/2-inch or larger tee or threaded connections for gauges.
Ambient Temperatures and Material Choices
Determine the ambient temperature range the gauge will be exposed to and verify the ambient temperature with the manufacturer’s literature for high ambient temperature and weather-resistant applications. Low ambient temperature conditions may require different models of gauges to be sure low temperatures will not affect the gauge readings.
Both the ambient temperature and pipeline fluid temperature will determine the material of the wet parts of pressure gauges. The materials can be specified as brass, stainless steel, nickel alloy, etc.
Another consideration is whether the pressure gauge will have a dry or liquid-filled case. The lower the ambient temperature, the more likely it is that a liquid-filled pressure gauge is the right choice. Pressure gauges in extremely cold environments, such as the Alaskan oil fields around the Arctic Circle, are filled with a special low-temperature-resistant silicone oil that does not thicken up very much at low temperatures and does not allow air and moisture into the case, preventing the internal parts from icing up and seizing.
If the fluid temperature will reach 140 F (60 C) or higher, use a stainless-steel gauge. This is because brass gauges are soldered, and solder begins to break down or weaken at temperatures above 140 F (60 C). Brass pressure gauges used on steam system applications have been known to fail.
The decision to choose brass pressure gauges vs. stainless-steel gauges is usually based on a first-cost decision. Brass gauges usually fail over time because steam exceeds 212 F; the temperature goes up with the steam pressure and will typically exceed the temperature threshold for solder. Stainless-steel gauges can withstand temperatures up to 392 F (200 C), depending on the configuration (with pigtails and snubbers).
Pressure and Temperature Gauge Applications
The first thing to do is determine the application for the pressure and temperature gauge. Gauges used for drinking water applications need to be lead-free, while process industries, such as refineries and pharmaceuticals, require industrial process gauges. Liquid nitrogen or cryogenic gas tanks call for a pressure gauge measuring differential and working pressure and is cleaned and rated for oxygen service. Gauges used in sanitary processes must have a hygienic design.
The highly aggressive gases used in the semiconductor industry means these applications need gauges with an ultra-high purity design. Most fire sprinkler applications require gauges to have Underwriter Laboratories and Factory Mutual approvals.
• Pressure gauge snubbers. Snubbers are typically installed with pressure gauges so that when valves or gauge cocks are opened or when pumps cycle on, the surge pressures spikes do not affect the pressure gauge. Sudden surges of pressure will cause a pressure gauge’s Bourdon tube to straighten out suddenly, causing the needle to spin rapidly. It can hit the pin that is the stop for the gauge pointer at very high velocity, causing the pointer to bend or causing it to fall off the center pinion of the gauge.
When I inspect buildings for plumbing problems, it is not unusual to see most pressure gauges with the needles bent or off the pinion. The reason this happens is because there is no snubber, and often not even a gauge valve or gauge cock. If a gauge is in a location that is subjected to a lot of pressure spikes or vibration, a gauge valve can be turned off and then only opened when a pressure reading is needed.
If a snubber is installed on a pressure gauge, it can be a brass or stainless-steel plug installed between the gauge and the pipeline with several tiny holes drilled to allow the line pressure to slowly pass through. However, surges or spikes in pressure will be snubbed or buffered so that the mechanical parts of the pressure gauge are not damaged.
For reliability and long service life in high-vibration applications (immediately downstream of pumps), use a liquid-filled gauge to dampen movement and protect the instrument’s internal mechanism from constant vibration of the pointer. Note that in high-pressure cycles (pulsation), liquid fill should be used in conjunction with a restrictor or a snubber.
The liquid fill also stabilizes the gauge pointer. Many installations without liquid-filled gauges have the pointer vibrate between two pressures marks. One recent inspection had the pressure gauge pointer vibrating between about 210 psi and 204 psi, so the two were added together and divided by 2 to get an average of 207 psi. The problem was that the pressure reading was in a 250 psi pressure gauge and was reading in the upper end of the pressure gauge dial.
Ideally, the working pressure should be displayed in the middle third of the pressure gauge. Since they were going to replace the gauge with a liquid-filled one, a good selection would be a 400-psi, liquid-filled gauge. Then the needle will read near the center of the gauge, and it should include a steady pointer for easy reading.
• Pressure gauge restrictors vs. pressure gauge snubbers. The difference between a restrictor and snubber is that a flow restrictor limits the flow rate through the orifice to a given flow rate, even as the pressure goes up, whereas a snubber is a single orifice with a needle valve or series of orifices that simply create a restriction of flow and pressure spikes.
Flow restrictors are a less-expensive option for gauges in applications with dynamic pulsation. However, they are limited based on the orifice size, and prone to clogging in debris-filled media, such as wastewater. Snubbers control the dynamic pulsations and pressure spikes much like restrictors, but they come in a wider range of sizes and are not as prone to clogging.
Snubbers are also more adjustable in the field with the use of interchangeable pistons or external adjustment screws, and this flexibility reduces downtime.
• Pressure gauge fluid or media. The fluid that the pressure gauge’s wetted parts are exposed to will determine the gauge material. A brass (copper alloy) gauge is suitable for water, air or other nonaggressive liquids or gases. Corrosive fluids such as hydrogen sulfide, ammonia, creosote and other harsh chemicals require corrosion-resistant materials, such as stainless steel or nickel-copper alloys.
For fluids that can clog gauge regulators or snubbers, opt for the addition of a diaphragm seal, which provides a physical barrier membrane between the fluid and the pressure instrument.
The fluid flowing in a pipeline with pressure gauges also affects the type of pressure gauge case filling used. Glycerin is the standard pressure gauge case fill fluid for nonoxidizing environments, while highly reactive media call for an inert oil such as halocarbon or other inert fluids.
• Pressure. Several types of pressure measurement are available; ensure that the correct pressure measurement is selected:
Gauge pressure (working pressure);
Absolute pressure;
Differential pressure.
Ideally, when using (mechanical) Bourdon tube-type gauges, the system working pressure range should read in the middle-third range of the pressure gauge.
For example, if I owned a high-rise building with a working pressure of 250 pounds/square inch (psi) on the pressure gauge near the ground floor, I would use a pressure gauge that goes up to 500 psi so the pointer on the gauge points to 250 psi and would be in the middle of the gauge. Near the top of the building where the working pressure is 40 psi, I would recommend a 100 psi pressure gauge and with 40 psi, the pointer will still be in the middle third of the gauge.
As opposed to a scenario where a contractor ordered all 500 psi gauges for all gauges in the building, then at the top floor, the pressure gauge would read 40 psi on a 500 psi gauge, and the pointer on the pressure gauge would be at less than 10% of the gauge range.
• Pressure gauge units. Determine what pressure gauge unit scale is desired. Gauges can be specified in a variety of measurement units such as psi, bar, kPa and inH2O. Gauges can be specified with custom pressure scales, such as dual scale (psi, bar), triple scale (psi, bar, kPa) or custom scales, based on the application.
Hot Water Temperature Maintenance Cables
Hot water temperature maintenance cables are a way of maintaining hot water within a given distance of the farthest fixture. When installed, check the following:
1. Check to see if the electrical circuits are ground-fault circuit breakers protected per the requirements in the electrical code.
2. Document the following:
• Manufacturer of temperature maintenance cables;
• Model number of cable;
• Temperature range/watts per linear foot of the cable;
• Minimum temperature to maintain on the plans/specs/
schedule.
3. Check the insulation type and thickness to see what thickness, type or insulation value is recommended by the temperature maintenance cable manufacturer.
4. Check the heating cable manufacturer’s installation manual vs. what is found in the actual installation; look for any deviations.
5. Check to see if a water management plan deals with hot water temperature maintenance cables. Electric heating cables can keep stagnant water in the ideal temperature range for Legionella bacteria growth (68 F to 122 F) for long periods, so a flushing routine should be established and documented for all hot water branches using hot water temperature maintenance cables.
6. Check to see if a periodic flush is required in the water management plan.
7. Check to see if a high-temperature heating/disinfection cycle is scheduled. Document the temperature, how long the cycle lasts and how often it is required to cycle.
8. Check to see if there is a periodic chemical disinfection and flush cycle scheduled. Is it documented in the water management plan?
9. How often does the flushing/purging of water from branch lines occur?
10. Review the water management plan for documentation of the electric heating cable system.
11. Are notes on system maintenance and repairs documented in the water management plan and signed off each time? (Note: Flushing should occur every three days maximum when seldom-used hot water branches are maintained in the Legionella growth range.)
Next month, we will continue with commissioning and troubleshooting domestic hot water systems.