Practical Process Engineering

#Book #Bobcock_Wilcox #41st_edition

Flow Control Magazine, March 2018

Import Heat Exchanger Data to Aspen EDR for Preliminary Design #HYSYS_Totorial_Videos

PSV Piping Expansions and Contractions #HYSYS_Tutorial_Videos

Using Activated EDR to Model a Rigorous Fired Heater in Aspen HYSYS #HYSYS_Tutorial_Videos

Using Column Internals for HYSYS Acid Gas Cleaning, #HYSYS_Tutorial_Videos

Two other issues: too narrow drip leg and protruding drip leg into the piping

On supervised startups, a valve is manually opened to drain and gravity alone is adequate to drain condensate. On automatic startups, however, there may not be sufficient differential pressure to provide flow through a steam trap orifice. In these cases, the drip leg must be sufficiently long to provide the necessary static head (dimension H) to push condensate through the trap. There are applications Where condensate must be lifted. The drip leg must provide ample volume for water to accumulate until there is sufficient pressure to elevate condensate to the level of the return (about 1 psi for every 2 ft of lift). The drip leg must be sized so the entire stream is drained away as it flows over the top of the opening. Partial drainage is likely when the common practice of “hot tapping” a small pipe into the steam main is used to provide a drip leg.

40- DRIP LEG (Collection Leg or Drain Pocket): A properly sized, enough wide piping called a drip leg (collecting leg, or drain pocket) is typically installed to help enable the efficient and effective removal of condensate from steam mains. Drip legs are, therefore, located at points where condensate may accumulate to allow for drainage by gravity down to a steam trap for proper discharge from the system. They are connected to the bottom of the lines with diameters large enough to promote collection. Condensate moves in a combination of ways, depending on the type of system under consideration. At startup, low pressure and a relatively high load create a gravity flow condition. High condensate loads, combined with moderate steam velocities, cause condensate to flow along the perimeter of piping. Under high velocity, low-load conditions, condensate actually becomes entrained in steam. Drip legs should be located every 30-50 m (100-160 ft). You can see a schematic of a trap draining drip leg at below picture.👇🏼👇🏼👇🏼

Some Important notes from ASME 31.1 33- 122.11.1 Drip Lines. Drip lines from piping or equipment operating at different pressures shall not be connected to discharge through the same trap. 34- 122.11.2 Discharge Piping. Trap discharge piping shall be designed to the same pressure as the inlet piping unless the discharge is vented to atmosphere, or operated under low pressure and has no stop valves. In no case shall the design pressure of trap discharge piping be less than the maximum discharge pressure to which it may be subjected. Where two or more traps discharge into the same header, a stop valve shall be provided in the discharge line from each trap. Where the pressure in the discharge piping can exceed the pressure in the inlet piping, a check valve shall be provided in the trap discharge line. A check valve is not required if either the stop valve or the steam trap is designed to automatically prevent reverse flow and is capable of withstanding a reverse differential pressure equal to the design pressure of the discharge piping. 35- If the trap discharges to atmosphere, make sure pipe it out to a safe location. 36- There are two types of errosion in steam/ condenstae pipings: impingments of liquid droplets and flashing errosion. The later may happen in steam trap outlet pipe transfering a two-phase stream of flash steam and condensate droplets. The flash steam has higher velosity and carries water droplets and liquid impingments happen again. The solution is to increase outlet size. 37- Take care of temperature fluctuations. Cold discharge can cause rapid condensation/ implosion of flash steam and it leads to cavitation errosion. 38- The distance between steam trap and pipe bends should be large enough not to allow impingments of oulet condensate droplets, especially for those traps having intermittent operation and blasting discharge. The return condesate pipes from steam traps discharge condensate to the top of the condenstae header. So, the return header should be large enough to minimize liquid droplet impingment. 39- A steam trap should be installed at the end of a pipe run (for example the main header) to drain condensate droplets.

hc Enthalpy of condensate at supply pressure, in (J/kg) hR Enthalpy of condensate at return line pressure, in (J/kg) Wc Condensate load, in (kg/h) VR Specific volume of steam at return line pressure, in (m3/kg) LR Latent heat of steam at return line pressure, in (J/kg) d Inside diameter of pipe, in (mm) (To be continued)

32- Based on the steam trap capacity and the design condensate load, determine the maximum distance between traps using the following formula where: MD=design condensate load per foot of pipe, lb/(ft*hr) MV=vendor steam trap capacity, lb/hr