4.1 Introduction
Steam and condensate pipes play an important role in industrial processes and boiler houses. They transport the steam and condensate to the processes or the heat exchangers and back again. Strict requirements have to be complied with regarding the laying and sizing of the pipes. There are so many factors to be taken into account that it is advisable to leave this design work to an expert.
The scope of this book is not adequate to cover every single aspect in detail. This chapter provides a rough overview of pipe-related topics in day-to-day practice.
4.2 Standards
Nominal diameters
The nominal diameter or size of a pipe, flange or valve is specified as a DN (standard diameter) value. For clarity, nominal diameters are used for calculations in this section. The connecting dimensions of a DN 40 valve exactly fit the connection flanges of a DN 40 pipe. The most popular pipe sizes are classified in a DIN EN 10220 series.
Nominal pressure ratings
Steam pipes are usually made of seamless steel. The choice of material and the pipe wall thickness are determined by the steam pressure and temperature. Each pipe has a nominal pressure rating (PN). This rating specifies the maximum permissible working pressure at 20 ºC. The most widespread pressure ratings are PN 10, 16, 25, 40, 63, 100, 160, 250, 320, etc. The same pressure ratings are also used for all valves. PN 40 generally means that the part will withstand a maximum pressure of 40 bar at 20 ºC. In addition to the pressure, the choice of material is also influenced by the process temperature. High temperatures have an effect on the material thickness required.
Kvs-value
As a rule, DN 40 does not necessarily mean that control or other valves has a 40 mm outlet. The Kvs-value, which provides a means of defining pressure drop, was introduced for this reason. The Kvs-value of a valve is defined as follows: the flow of water at 20 °C in m3/h and a density of 1ς, which gives a pressure drop of 1 bar.
Other standards
DIN EN standards are the norm in Germany. DIN stands for "Deutsches Institut für Normung" (German Institute for Standardisation) while EN denotes a recognised European standard. ANSI and ASME standards are also common in certain branches of the process industry (petrochemicals, offshore and chemical plants).
4.3 Determination of the pipe diameter
Amongst other things, the pressure drop in a pipe is dependent on the volume flow, the flow velocity and the viscosity of the fluid. The more steam flows through a pipe with a particular nominal diameter, the higher the friction on the pipe wall. In other words, the higher the steam velocity, the greater the friction against the pipe wall, and the greater the pressure drop. If a pipe is used to transport superheated steam to a steam turbine, the pressure loss should be as small as possible. However, this kind of pipe is more expensive than ordinary pipes; a larger diameter immediately puts up the cost dramatically. The investment calculation is based on the payback period for the investment amount compared to the profit from the turbine output.
This calculation is always based on the peak load of the turbine rather than to the average load. If 1000 kg of steam is discharged during a 15-minute period at peak load, for instance, the pipe must have a capacity of 60/15 x 1000 = 4000 kg/h.
Calculation
Chapter 6.0 Condensate Management explains how to determine the diameter of a condensate pipe. The calculations for steam, air and water pipes rest on practically the same assumptions. These calculations are therefore described in the following in order to round off the topic.
The fundamental formula for calculating the diameter is as follows:
where:
Q = Steam, air or water flow rate in m3/s
D = Diameter of the pipe in m
v = Maximum velocity in m/s
In practice, it is advisable to specify the flow rate in m3/h and the pipe diameter in mm. The above formula must therefore be adapted as follows in order to calculate the required diameter:
where:
D = Diameter of the condensate pipe in mm
Q = Flow rate in m3/h
v = Maximum velocity in m/s
Pipe calculations are always based on the volume flow (m3/h) rather than the mass flow (kg/h). If only the mass flow of the steam is known, the steam tables must be consulted in order to convert from kg/h to m3/h via the specific volume
Example:
The specific volume of saturated steam at 11 bar is 0.1747 m3/kg. The volume flow of 1000 kg/h of saturated steam at 11 bar is therefore 1000 x 0.1747 = 174.7 m3/h. If the same amount of superheated steam is present at 11 bar and 300 ºC, the specific volume is 0.2337 m3/kg and the volume flow 233.7 m3/h. In other words, a steam pipe that is suitable for transporting saturated steam cannot necessarily be used to carry the same amount of superheated steam
The pressure is additionally required to calculate air and other gases. Compressor manufacturers specify compressor capacities in mo 3/h, which means atmospheric m3 at 0 ºC.
If the compressor capacity is 600 mo 3/h and 6 bar compressed air is used, the volume flow is 600/6 = 100 m3/h; this is also the basis for the pipe calculation.
Maximum flow velocity
The maximum flow velocity in a pipe system is influenced by several factors.
- Plant costs: a low velocity means a large diameter.
- Pressure loss: a high velocity means a small diameter and more pressure loss.
- Wear: a high velocity means more erosion, especially with condensate.
- Noise: a high velocity means more noise, e.g. due to steam pressure reducing valves.
The table below shows the recommended flow velocities for various media.
| Medium | Function | Velocity in m/s |
|---|---|---|
Steam | Less than 3 bar | 10 - 15 |
| 3 - 10 bar | 15 - 20 | |
| 10 - 40 bar | 20 - 40 | |
| Condensate | Filled with condensate | 2 |
| Condensate-steam mixture | 6 - 10 | |
| Feedwater | Suction pipe* | 0.5 - 1 |
| Discharge pipe | 2 | |
| Water | Drinking water | 0.6 |
| Cooling water | 2 | |
| Air | Compressed air | 6 - 10 |
| * Suction pipe of the feedwater pump: the low velocity means a smaller pressure drop, so that cavitation is avoided at the inlet of the feed pump | ||
Fig. 4-1: Recommended flow velocities
Examples:
a) Water
Calculation of the pipe diameter for 100 m3/h of water at v = 2 m/s.
=133mm. Selected nominal diameter: DN 125 or DN 150.
b) Compressed air
Calculation of the pipe diameter for 600 mo 3/h of compressed air at 5 bar and a velocity of 8 m/s.
Conversion from 600 mo 3/h to actual m3/h:
= 72mm. Selected nominal diameter: DN 65 or DN 80.
The decision in favour of DN 65 or DN 80 is determined by the purpose for which the water or air is intended. It should be noted that the diameter calculation is based on the mean value and takes no account of sporadic peak loads.
c) Saturated steam
Calculation of the pipe diameter for 1500 kg/h of saturated steam at 16 bar and a velocity of 15 m/s.
According to the steam table, the specific volume of 16 bar saturated steam vg is 0.1237 m3/kg.
Once again, the decision for DN 65 or DN 80 depends on the potential peak loads. Future developments may also have to be considered.
d) Superheated steam
If the steam in this example is superheated to 300 ºC, the specific volume changes to vg = 0.1585 m3/kg.
, in other words DN 80.
The nomogram in Fig. 4-9 (Page 76) shows how to determine the pipe diameter for water or air graphically without calculations. The nomogram in Fig. 4-10 (Page 77) shows this process for saturated steam and superheated steam.
e) Condensate
If the pipe is a condensate pipe without any flash steam, the diameter calculation is identical to that for water.
If hot condensate is transported in a condensate pipe after passing through the steam trap, this condensate is flashed. Chapter 6.0 Condensate Management explains how to read off the percentage of flash steam.
The volume of the residual water is so small in relation to the volume of the flash steam that it can be disregarded.
Calculation of the diameter of a condensate pipe with 1000 kg/h of condensed steam at 11 bar (hf = 781 J/kg), flashing to the 4 bar condensate system (hf = 604 kJ/kg, vg = 0.4622 m3/kg and hfg = 2133 kJ/kg).
The percentage of flash steam is as follows:
The flash steam flow rate is as follows: 1000 x 0.083 = 83 kg/h or 83 x 0.4622 = 38 m3/h. The percentage of flash steam by volume is approximately 97 %.
The pipe diameter for the mixture at a velocity of 8 m/s is as follows:
The percentage of flash steam for an atmospheric condensate system (vg = 1.694 m3/kg) is as follows:
In this case, the pipe diameter is as follows:
Velocity of the mixture
There is some disagreement amongst experts when it comes to the maximum permissible flow velocities for mixtures of flash steam and condensate. Velocities between 15 and 20 m/s are often mentioned in the literature. Based on practical experience, however, these values are far too high. The reason for this is that a condensate pipe transports 95 percent flash steam by volume and 5 percent water. The mixture has a velocity of 20 m/s. Drops of water that are entrained in the flash steam hit the first bend in the pipe at the same velocity as this steam (20 m/s, equivalent to 72 km/h!). A small amount of condensate collects at this point. The free flow is impaired as a result of this backing-up. The condensate gradually builds up, then suddenly shoots through to the next pipe bend as a plug, etc.
If a leak occurs in a condensate pipe, it is almost certain to be at a bend. The thin area in the bend is situated upstream at three-quarters of the elbow.
In addition, the velocity of 72 km/h is based on continuous flow, even though all steam traps – with the exception of float-type traps – operate intermittently. It is not uncommon for a steam trap (especially a bimetallic type) to remain shut for more than half the time!
Actual velocities of 100 to 140 km/h are nothing unusual in a condensate system.
Practice has shown that condensate drainage problems are usually attributable to pipe diameters that are too small and only rarely to the type of steam trap selected. A velocity of 6-8 m/s is recommended in the condensate pipe downstream of a steam trap.
4.4 Expansion of steam pipes
Introduction
When steam pipes are designed, a certain expansion must be allowed for owing to the temperature variations when starting up or shutting down the plant. The coefficient of expansion for steel is 0.012 mm/m ºC. This means that the pipe expands 0.012 mm per metre length for every one-degree rise in temperature. If a 10 bar saturated steam pipe with a length of 25 m is heated from 15 ºC to 180 ºC, it will expand (180 - 15) x 0.012 x 25 = 50 mm.
A superheated steam pipe at 450 ºC installed between the steam boiler and the turbine expands 5.4 mm per metre length. The forces of expansion are so enormous that the turbine would be literally uprooted from its foundations if nothing was done to accommodate the expansion.
Every material has a unique coefficient of expansion: copper, for example, has a coefficient of 0.016 mm/m ºC while stainless steel has a coefficient of 0.019 mm/m ºC.
Compensating expansion
In the absence of suitable measures to compensate for expansion, considerable stresses and forces will act in or on the apparatus, pipes and valves. The pipes can be distorted to such an extent that they break away from their points of support.
Pipes must be laid so that they can expand or contract freely as they heat up and cool down. Steps must also be taken to prevent axial movement. The risks arising from inexpert installation tend to be very high and it is therefore advisable to trust the planning of your pipe system to a qualified specialist.
A few general recommendations for planners of pipe systems are provided in the following.
- A straight pipe section must never be installed between two anchors (refer to Fig. 4-2).
Fig. 4-2: Pipe with anchors only
Pipes should be installed so that there is only ever an anchor (fixed point) on one side; all other supports should be sliding (refer to Fig. 4-3).
Fig. 4-3: One anchor point, all other supports sliding
- If a pipe is installed with a 90º bend, both sections must be able to expand freely. The distance between the anchor and the bend must be sufficient to accommodate the expansion (refer to Fig. 4-4).
Fig. 4-4: Design with a 90º bend
- If a pipe is installed with branches, the anchors in the main pipe and in the branches must be located relatively close together. Branches in steam pipes must exit on the top side. Similarly, condensate pipes must enter on the top of the return pipe (refer to Fig. 4-5).
Fig. 4-5: Design with branches
- Fig. 4-6 shows a few designs of anchor and sliding points.
Fig. 4-6: Anchor and sliding point designs
If there is not enough room to expand in the longitudinal direction, expansion loops or bellows can be used (refer to Fig. 4-7). These loops are usually right-angled, although lyre loops are sometimes also used.
Fig. 4-7: Bellow Compensator
Assuming there is sufficient space, the expansion loops should be mounted horizontally. If not, they should be fitted vertically – with a drain point either ahead of, or preferably in, the rising pipe bend. The drain point consists of a drain port with a steam trap (refer to Chapter 5.0 Pipe Drainage). It is important to make sure that the expansion of the main pipe is not inhibited by the condensate pipe.
4.5 Heat loss via pipe supports
It has become standard practice in the last few years to weld roller supports to a pipe. However, this solution ignores the fact that a relatively large amount of heat is lost via welded rollers. As a result, the average temperature of the roller supports is only 30 to 40 ºC less than the steam temperature in the pipe!
It is therefore advisable to design the supports in a new plant with brackets. A glass fibre mat should then be inserted between each bracket and the pipe.
4.6 Heat transmission losses in uninsulated pipes and valves
The heat transmission losses in uninsulated indoor and outdoor pipes are given in tables for various temperature differences (between the ambient and process temperatures).
Fig. 4-8: Uninsulated indoor pipes
Example:
An uninsulated DN 150 pipe installed in a building loses 1.6 kW of heat per metre length if there is a temperature difference of 200 ºC. A DN 150 valve in the same pipe system loses 2 x 1.6 = 3.2 kW
Based on a heat generating efficiency of 90 % and 8000 hours operation annually, the valve losses heat equivalent to:
Assuming a gas price of €0.20 /m3, this represents a yearly loss of €650.- .
Since it costs around €240.- to insulate a valve, the investment pays off in approximately six months!
Example:
An uninsulated DN 150 pipe installed in the open loses 4.5 kW of heat per metre length if there is a temperature difference of 200 ºC. A DN 150 valve in the same pipe system loses 2 x 4.5 = 9 kW. Based on a heat generating efficiency of 90 % and 8000 hours operation annually, the valve loses heat equivalent to:
of natural gas.
Assuming a gas price of €0.20 /m3, this represents a yearly loss of €1820.- .
The saving that can be achieved by insulating the pipe is about 80 % – in this case 0.8 x €1,820.- = €1,450.- .
Since it costs around €240.- to insulate a valve, the investment pays off in two months!
Determination of the nominal diameter of water and air pipes
Example: (refer to Fig. 4-9).
a) Water:
Pipe for 100 m3/h with a flow velocity v = 2 m/s.
Result: DN 125.
b) Compressed air:
Pipe for 600 m3/h with an air pressure of 6 bar and a flow velocity v = 8 m/s.
600 m3/h is equivalent to 600/6 = 100 actual m3/h.
Result: DN 65.
Fig. 4-9: Nomogram for determining the nominal diameter of water and air pipes
Determination of the nominal diameter of, and flow velocity for, steam pipes
Fig. 4-10 can be used to determine the flow velocity for a steam pipe based on the steam temperature and pressure as well as the nominal diameter. Conversely, the nominal diameter of the steam pipe can be derived from the nomogram and the flow velocity.
a) Determination of the flow velocity (example):
Superheated steam, 30 t/h, 300 ºC, 16 bar, DN 200.
Follow the dashed line horizontally from 300 ºC up to the 16 bar line. Then continue vertically downward from this point as far as the 30 t/h flow rate line and from there leftward to the DN 200 nominal diameter line. Next, continue vertically to the flow velocity lines and read off the velocity at 43 m/s.
b) Determination of the nominal diameter (example):
Superheated steam, 30 t/h, 250 ºC, 12 bar, flow velocity 30 m/s.
Follow the dashed line horizontally from 250 ºC up to the 12 bar line. Then continue vertically downward from this point as far as the 30 t/h flow rate line. Plot a horizontal line to the left and right, then starting at 30 m/s on the flow velocity scale follow a vertical line until it intersects the plotted horizontal line. The nominal diameter can be read off at the point of intersection: DN 300.
For saturated steam, start at the pressure curve in the top part of the nomogram and follow this curve up to the saturated steam curve, then continue downward to the bottom part of the nomogram.
Determination of the nominal diameter of condensate pipes
The nominal diameter of condensate pipes can be determined directly using the table in Fig. 4-11.
Fig. 4-11: Determination of the nominal diameter of condensate pipes
To determine the nominal diameter of a pipe using the value obtained with the table, this value must be multiplied by the corresponding flow rate factor in the table below.
The table is based on the formula explained in Chapter 6.0 Condensate Management as well as on a flash steam flow velocity of 10 m/s and a condensate flow rate of 100 kg/h.
where:
D = Diameter of the condensate pipe in mm
Q = Flow rate in m3/h
v = Maximum velocity in m/s
Example 1:
Condensate flow rate: 500 kg/h
Steam pressure: 4 bar
Back pressure: atmospheric (1 bar)
Nominal diameter: ?
In Fig. 4-11, look up the value for 4 bar steam pressure in the 1 bar back pressure column = 22.2 bar (abs). Read off the factor 2.2 under 500 kg/h in the bottom table. Multiply the value 22.2 by the factor 2.2. Result: 22.2 x 2.2 = 48.8 mm, in other words DN 50. If the required velocity is less than 10 m/s, e.g. 5 m/s, the calculated diameter must be multiplied by 
In this case, DN 50 becomes DN 80, for example.
Example 2:
Condensate flow rate: 1500 kg/h
Steam pressure: 9 bar
Back pressure: 2 bar in the condensate system
Nominal diameter: ?
In Fig. 4-11, look up the value for 9 bar steam pressure in the 2 bar back pressure column = 18.2 bar (abs). Read off the factor 3.9 under 1500 kg/h in the bottom table.
Multiply the value 18.2 by the factor 3.9. Result: 18.2 x 3.9 = 71 mm. Depending on the conditions on site (long pipe, varying peak load), select either DN 80 or – if the pipe is short with a constant condensate flow rate – DN 65.