Pipe Sizing for Commercial and Industrial Natural Gas at Operating Pressures 75 mbar

Transcription

1 Pipe Sizing for ommercial and Industrial Natural Gas at Operating Pressures 75 mbar These notes form part of the gas safety training programme from RD Training (Midland) Ltd. They are for information only and must not be used as a substitute for normative documents or manufacturers instructions. RD Training (Midland) Ltd. accepts no responsibility for any consequences of their use. Operating Pressure (OP) Operating pressure is defined by the Gas Safety (Installation and Use) Regulations as the pressure at which a gas appliance is designed to operate. Gas pipework systems must provide all appliances with gas at their correct operating pressure. Undersized pipes give low pressure which could cause appliance malfunction. Maximum permissible working pressure drop ccording to IGEM/UP/2 Edition 3 the loss in working pressure from gas meter outlet to appliance inlet should not be more than: 1 mbar in installations with operating pressure of 25 mbar or less; or 10% of the operating pressure in systems with operating pressure of more than 25 mbar. Gas velocity High gas rates in small pipes cause gas to flow at high speeds that can erode components and fittings. The speed of gas in pipework should not exceed 20 m / s in unfiltered systems, and it should not exceed 40 m / s in systems with filters up to 250 micron (0.25 mm). In a steel pipe of nominal bore d with a gas flow rate Q (m 3 / s), if the operating pressure is 20 mbar, gas velocity (m / s) 348 x Q d d More generally, for systems of operating pressure OP, gas velocity (m / s) x Q d d (OP ) 2015 RD TRINING (MIDLND) LTD. 1

2 auses of working pressure loss in gas pipework Working pressure loss is caused by friction from pipe walls and is determined by: pipe diameter smaller diameters give more pressure loss pipe length longer pipes give more pressure loss gas flow rate higher gas loads give more pressure loss fittings and bends restrictions and changes of direction increase pressure loss pipe condition dents, excess solder etc. restrict gas flow and lower pressure pipe material steel, copper, and polyethylene (PE) have different frictional losses The only factor fully controllable by the designer is pipe diameter. Note 1: industrial gas systems are often running at maximum load, giving less margin of error than installations where there is more diversity (such as domestic systems). Note 2: when gas is not flowing its pressure is not affected by friction. Standing (static) pressure therefore cannot be used to check pipe size. Why bigger pipes lose less working pressure Bigger pipes have more open area where gas can flow without friction from the pipe wall: 25 mm gas pipe 50 mm gas pipe Diameter = 2 x bigger Pipe wall = 2 x bigger Open area = 4 x bigger Pipe sizing in the role of the system designer Pressure loss is unavoidable but can be kept to an acceptable level by correct pipe sizing. The designer must ensure safe and efficient operation of any connected appliance under all normal operating conditions and allow for any probable future expansion. On the other hand, bigger pipes increase installation and maintenance costs. They also increase the Installation Volume (IV) and Purge Volume (PV) of the system, making tightness testing more time consuming and purging more hazardous. The goal of pipe sizing is to choose pipes that are neither too big nor too small RD TRINING (MIDLND) LTD. 2

3 Using a pipe sizing chart Table 1 on page 12 has been adapted from IGEM/UP/2 Edition 3. It tells you the gas flow rate (in cubic metres per hour) that a particular diameter and length of pipe can carry while giving a 1 mbar pressure drop. Boiler Gas rate 10.5 m 3 / h 16 metres Steel gas pipe Using Table 1, try out increasing diameters of steel pipe, looking across to the length needed to see what gas rate it can carry with a 1 mbar drop. Note: 16 m length is not in the table so go across to 20 m; do not under-estimate to 15 m. The smallest steel pipe that can carry 10.5 m 3 / h over 16 metres with no more than a 1 mbar drop is 32 mm RD TRINING (MIDLND) LTD. 3

4 orrecting pipe lengths for the pipe sizing chart Systems with more than one appliance are divided into legs which can be sized individually. onsider the following installation: 7 metres B 8 metres D Steel pipe OP = 20 mbar 4 metres Gas leaves the meter and travels along leg B. t B, some of the gas branches off to appliance, and some continues to appliance D. The total pressure loss across either route should not exceed 1 mbar. We need to allocate pressure losses for the individual pipe legs. The simplest way is to size each leg so that it loses 0.5 mbar. Gas will then lose 0.5 mbar across B and then a further 0.5 mbar across either B or BD. Unfortunately, Table 1 always gives the pipe size for 1 mbar loss, whereas here we only want to lose 0.5 mbar. To solve this, instead of using the actual length, use length pressure loss when using Table 1. Here s why it works: Leg B is 7 metres long and we want 0.5 mbar drop across it. pressure loss = = 14 metres * Using this in Table 1 gives the pipe size for 1 mbar drop across 14 metres. The actual pipe is only half this length so there will be only half the drop (0.5 mbar). This works for any pressure loss, including losses of more than 1 mbar which may be encountered with operating pressures over 25 mbar. * The actual unit is metres per millibar 2015 RD TRINING (MIDLND) LTD. 4

5 llocating pressure losses for pipe legs In the example below there are appliances at, E, and F: B D F Steel pipe OP = 20 mbar E Dividing 1 mbar between the three legs on the main run (B, BD and DF) gives 1 3 = mbar loss each. It s easier to avoid recurring decimals so choose simpler numbers which add up to 1, for example B = 0.4 mbar, BD = 0.3 mbar and DF = 0.3 mbar: 0.4 mbar B 0.3 mbar D 0.3 mbar F E We can now calculate the losses for the remaining branches, B and DE: B: the gas has lost 0.4 mbar by the time it gets to B. This leaves a further 0.6 mbar to lose to. DE: the gas has lost = 0.7 mbar at D. This leaves a further 0.3 mbar to lose to E: 0.4 mbar B 0.3 mbar D 0.3 mbar F 0.6 mbar E 0.3 mbar For regular pipe systems like this the easiest way to allocate pressure losses is to divide the permitted pressure loss by the number of legs on the main line. For example, a system with four appliances could have 1 4 = 0.25 mbar loss across each leg of the main run. It is then simple to work out the pressure losses of the branches so that they total 1 mbar to each appliance RD TRINING (MIDLND) LTD. 5

6 llocating pressure losses in irregular pipework systems In real-world installations there is often an extra-long pipe leg, or a leg that supplies an appliance with a much higher heat input than others. These legs suffer more working pressure loss. If you allocate pressure losses evenly they may end up being bigger than upstream pipes. To prevent this you can give them a larger share of the total drop. In the example below, leg BD is much longer than the others so we have allowed it more of the total drop. This will help keep its diameter small. 0.2 mbar B 0.6 mbar D 0.2 mbar F Steel pipe OP = 20 mbar 0.8 mbar E 0.2 mbar You can distribute pressure losses any way you wish as long as their total to each appliance does not exceed the maximum permitted amount. alculating gas flow rate in pipe legs B D F 15 m 3 / h 8.2 m 3 / h E 9.3 m 3 / h Leg B has to carry gas for all three appliances, = 32.5 m 3 / h Leg B only has to carry gas for the appliance at = 8.2 m 3 / h Leg BD has to carry gas for the appliances at E and F, = 24.3 m 3 / h Note: you can also calculate this as = 24.3 m 3 /h Leg DE only has to carry gas for the appliance at E = 9.3 m 3 / h Leg DF only has to carry gas for the appliance at F = 15 m 3 / h 2015 RD TRINING (MIDLND) LTD. 6

7 Treating fittings as additional lengths of pipe When gas flow changes direction or diameter in a fitting or bend it loses more pressure than in a straight run of pipe. You must therefore include fittings and bends in your calculations. One way of doing this is to treat them as additional lengths of pipe. Table 2 on page 12 tells you how much length to add for several types of fitting. Unfortunately, the additional length depends on the diameter of the fitting, which we don t know at first. It will be the same size as the pipe going into it, which we must estimate before adding in the fittings and repeating the calculations. quick method of pipe sizing The full method of pipe sizing can be time-consuming because of the way fittings have to be added in and the calculations repeated. For quick estimates, or in situations where the number of fittings is not known, you could simply add a percentage to all pipe lengths to account for fittings. The calculations will be much simpler and will need only one pass. Unfortunately, the percentage increase for fittings is difficult to estimate. For a system with between two and four fittings on each leg a value of 30% should work. Note: it needs to be borne in mind that this method should not be used where critical accuracy is needed, particularly where there is a high number of fittings. The full method of pipe sizing Using the calculation sheet on page 11 and a line diagram of the system: 1. llocate pressure losses for all pipe legs (see page 5). 2. Determine the gas flow rate in each leg (see page 6). 3. For each leg: a. alculate length pressure loss. b. the pipe diameter using Table 1. c. alculate the additional length from fittings using Table 2. d. dd the additional length to the actual length. e. Using this new length, repeat from step 3a until the diameter does not increase. Note: any reducer fittings needed should be included in the calculations RD TRINING (MIDLND) LTD. 7

8 Worked example pipe sizing question alculate steel pipe sizes for the following natural gas installation (OP = 20 mbar). L B L L L X 15 m 18 m D Radiant Tube 7 m 3 /h X = tee L = elbow 5 m Boiler 15 m 3 /h 1. llocate the pressure losses: B = 0.5 mbar, B = 0.5 mbar, BD = 0.5 mbar 2. Determine the gas rates: B = 22 m 3 / h, B = 15 m 3 / h, BD = 7 m 3 / h 3. For B: ctual + Fittings B elbows + 1 through tee = 3 x = = 20.3 Using the new length of 20.3 m repeat the method on a new line: ctual + Fittings B The second estimate is the same as the first, so 50 mm is the final pipe size for B RD TRINING (MIDLND) LTD. 8

9 For B: ctual + Fittings B tee into branch (50mm) + 1 reducer (50 to 32mm) = = = 11 Note that this leg estimate is 32 mm, which is two sizes down from B (50 mm), so an additional Type 3 reducer fitting is included. Using the new length of 11 m repeat the calculations: B tee into branch (50mm) + 1 reducer (50 to 40mm) = = 5 ctual + Fittings = 10 With fittings the pipe size estimate has gone up to 40 mm. This leg now only reduces by one size from B so you can change the reducing fitting to Type 1, which has a shorter additional length than before. This means you can be sure that 40 mm is big enough for B. For leg BD: BD reducer (50 to 32mm) = 1.5 ctual + Fittings = 19.5 Note that this leg reduces two sizes from B, so an additional Type 3 bush fitting is included. The through-tee was counted for leg B, so do not add it again here. BD ctual + Fittings This is the same as the previous estimate, so 32 mm is the final pipe size for leg BD. The pipe sizing is now complete RD TRINING (MIDLND) LTD. 9

10 If the length of a pipe leg goes off the scale of Table 1 (>250 metres) Divide the length by 2 Multiply the gas rate by the square root of 2 For example, a steel pipe has length 295 metres. The gas flow rate is 84 m 3 / h. This pipe length exceeds the longest length in Table 1. alculate length 2 = = metres. Gas rate x 2 = 84 x = m 3 / h. Looking up these figures in Table 1 shows that 150 mm steel pipe will suffice. If it s closer to round down to a pipe length than to round up in Table 1 Divide the length of the pipe leg by the nearest length down in Table 1 Find the square root of this number Multiply this by the gas rate and use the result instead of the actual gas rate Use the rounded down length and the modified gas rate in Table 1 For example, a steel pipe is 52 metres long and must carry a load of 33 m 3 / h. In Table 1, when the exact length you want is not in the chart, you would normally look across to the next length up. In this case it is 75 m, a long jump up from the real length of 52 metres. This could cause oversizing. Using the above method, calculate = Taking the square root gives Multiplying this by the gas rate gives x 33 = m 3 / h. Using this and the rounded down length to look up the pipe size in Table 1 (50 m at m 3 / h) gives a pipe size of 65 mm. If you had used 75 m at 33 m 3 / h it would have given 80 mm, which is too big. To estimate pressure loss across a pipe leg If Q = gas flow rate d = nominal bore of the pipe L = length of the pipe including allowance for fittings pressure loss ( d ) x Q x Q d d d d d x L For example, 21 metres of 50 mm steel pipe carrying 22 m 3 / h gas would have a pressure loss of: ( ) x 22 x x mbar 2015 RD TRINING (MIDLND) LTD. 10

11 PIPE SIZING LULTION SHEET ctual + Fittings 2015 RD TRINING (MIDLND) LTD. 11

12 TBLE 1 GS FLOW RTES (m 3 /h) GIVING 1 mbar PRESSURE DIFFERENTIL # = higher flow rates exceed a gas speed of 20 m / s TBLE 2 EQUIVLENT LENGTHS OF FITTINGS NOMINL PIPE SIZE EQUIVLENT LENGTH arbon and Stainless Steel opper PE Type 1 Type 2 Type 3 Type 4 Type 5 45 º bend, 90 º long bend, Bush and socket (one size change) Through-tee, 90 º bend, Full bore valve, Union, dapter, Flange joint 90 º elbow, Bush and socket (more than one change of size) Tee entering from a branch Tee entering into a branch RD TRINING (MIDLND) LTD. 12