Engineering Manual o.. - HVAC.Amickracing

LINE PRESSURE IN PSIVALVE SELECTION AND SIZINGEXAMPLE:A valve with a close-off rating of 10 psicould have 40 psi upstream pressure and30 psi downstream pressure. Note that inapplications where failure of the valve toclose is hazardous, the maximum upstreampressure must not exceed the valve closeoffrating, regardless of the downstreampressure.The valve close-off rating is independent of the actualvalve body rating. See definition of BODY RATING(ACTUAL) in this section.Close-off rating of three-way valves: The maximum pressuredifference between either of the two inlet ports andthe outlet port for mixing valves, or the pressuredifference between the inlet port and either of the twooutlet ports for diverting valves.Critical pressure drop: See Pressure drop (critical).Pressure drop: The difference in upstream and downstreampressures of the fluid flowing through the valve.Pressure drop (critical): The flow of a gaseous controlled fluidthrough the valve increases as the pressure dropincreases until reaching a critical point. This is thecritical pressure drop.Any increase in pressure drop beyond the criticalpressure drop is dissipated as noise and cavitationrather than increasing flow. The noise and cavitationcan destroy the valve and adjacent piping components.Body rating (nominal): The theoretical pressure rating,expressed in psi, of the valve body exclusive ofpacking, disc, etc. The nominal rating is often cast onthe valve body and provides a way to classify the valveby pressure. A valve of specified body material andnominal body rating often has characteristics such aspressure-temperature ratings, wall thickness, and endconnections which are determined by a society suchas ANSI (American National Standards Institute).Figure 2 shows ANSI pressure-temperature ratings forvalves. Note that the nominal body rating is not thesame as the actual body rating.Body rating (actual): The correlation between safe, permissibleflowing fluid pressure and flowing fluid temperatureof the valve body (exclusive of the packing, disc, etc.).The nominal valve body rating is the permissiblepressure at a specific temperature.EXAMPLE:From Figure 2, a valve with an ANSI ratingof 150 psi (ANSI Class 150) has an actualrating of 225 psi at 200F.Maximum pressure and temperature: The maximumpressure and temperature limitations of fluid flow thata valve can withstand. These ratings may be due tovalve packing, body, or disc material or actuatorlimitations. The actual valve body ratings areexclusively for the valve body and the maximumpressure and temperature ratings are for the completevalve (body and trim). Note that the maximum pressureand temperature ratings may be less than the actualvalve body ratings.EXAMPLE:The body of a valve, exclusive of packing,disc, etc., has a pressure and temperaturerating of 125 psi at 355F. If the valvecontains a composition disc that can withstanda temperature of only 240F, then thetemperature limit of the disc becomes themaximum temperature rating for the valve.40030025020015010050ANSICLASS 125212 o FANSICLASS 150ANSICLASS 250ANSI CLASS150(STEAM)275 o F337 o F00 50 100 150 200 250 300 350 400FLUID TEMPERATURE IN o FNOTES:1. FOR HIGH FLUID TEMPERATURES, THE VALVE AND/ORPIPING SHOULD BE INSULATED TO PREVENT AMBIENTTEMPERATURES FROM EXCEEDING ACTURATOR RATINGS.M12224Fig. 2. ANSI Pressure-Temperature Ratings for Valves.VALVE TYPESBall valve: A ball valve has a precision ball between two seatswithin a body (Fig. 3). Ball valves have several portsizes for a give body size and go from closed to openwith a 90 degree turn of the stem. They are availablein both two-way and three-way configurations. ForHVAC applications, ball valve construction includesbrass and cast iron bodies; stainless steel, chromeplated brass, and cast iron balls; resilient seats withvarious temperature ratings.ENGINEERING MANUAL OF AUTOMATIC CONTROL404