Engineering Manual o.. - HVAC.Amickracing

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONSHIGH TEMPERATURE WATER SAFETYA well designed HTW system with proper installation ofpiping to prevent undue stress rarely fails.HTW drops in temperature very rapidly with a minor, lowmass, flow leak into space. In low or medium temperature waterthe escaping fluid remains at nearly the same temperature. HTWflashes to steam and the turbulent mixing of liquid and vaporwith the room air rapidly drops the fluid temperature below100°C and within a short distance to 52 to 60°C.Minor leakage at valves is usually imperceptible except fordeposits of scale at the point of leakage. Valves should beinspected periodically and scale removed from valve stems toprevent damage to packing and to allow free valve movement.HTW CONTROL SELECTIONThe features of good temperature control for low and mediumtemperature water heating systems also apply to hightemperature water heating. Special considerations are neededfor controls to withstand the temperatures and pressuresencountered in a HTW system. The large temperature differencebetween HTW and the heated fluid (air, water, or steam) meansthat most of the control selection precautions are to providefail safe operation. Careful consideration must be given to whathappens to the secondary fluid of a converter and its controlswhen flow is reduced or stopped, or equipment is shutdown.Secondary fluid can overheat immediately after or duringprolonged shutdown. Controls near a HTW supplied coil canbe damaged by high temperature if airflow stops.Controls can be pneumatic, electric, or digital. The low massof electronic temperature sensors (thermocouple or resistancebulb) provides faster response than fluid-filled or bimetalelements. Pneumatic actuators generally have the fasterresponse, higher temperature ratings, and greater reliabilityrequired for HTW applications.Standard commercial quality controls with integral(automatic reset) and derivative (rate) action are satisfactoryfor most applications when used with industrial quality valves.An industrial controller or a digital system may be requiredwhere local recording of a control value is specified. The controlsystem selected should:1. Function from zero to full load.2. Close the HTW valve if there is failure in the controlsystem, loss of power or air, or shutdown of the systembeing controlled.HTW VALVE SELECTIONValves for HTW must be selected to ensure suitability forhigh temperature and pressure conditions. A control valve musthave wide rangeability (50 to 1 is desirable) and the ability toposition accurately and hold that position. Rangeability is theratio of maximum flow for a valve to the minimum controllableflow. These requirements dictate use of industrial quality valves.For additional information on rangeability see the ValveSelection and Sizing section.VALVE STYLESingle seated valves are recommended because of their tightshut-off and availability with equal percentage and reducedcapacity (Kv) trim.Double seated valves are not recommended because they donot provide tight shut off. High velocity leakage past the plugof a double-seated valve causes greater erosion to the seats thana single-seated valve.Three-way valves are not recommended because they are notavailable with equal percentage flow characteristics andtherefore do not provide the required control stability. Theyalso are not available with reduced Kv trim for accurate sizing.Valve bodies should be flanged although threaded bodies canbe used for sizes of ISO 32 mm or less. Weeping and flashingat threads is likely but usually not visible except for depositsleft behind. If threaded valves are used in the HTW lines, useadjacent bolted flanges not union fittings.VALVE BODY MATERIALSCast steel bodies with 2000 kPa or 4000 kPa body ratingsare recommended. These have temperature ratings above400°C, the limit for packing materials. Manufacturers literaturelists actual pressure and temperature ratings.Bronze bodies are not recommended because the maximumtemperature limit is 200°C. In addition, since HTW piping issteel, galvanic action between bronze and steel can be seriousat the elevated temperatures encountered.Cast iron bodies are not recommended because maximumoperating conditions are limited to 208°C at 1700 kPa, whichare too close to operating conditions.ENGINEERING MANUAL OF AUTOMATIC CONTROL375