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HOW TO PURGE WITH NITROGEN
Nitrogen purging is easily adapted to any process installation. Different Different methods are used depending on the type and shape of the equipment to be purged and on the location of the purging inlets and outlets. 1. Displacement Purging
This method is used for equipment with simple cross sections such as piplines. The volume of nitrogen required corresponds to the physical volume of the pipe. In many piplines, a rubber scraping piston, or "pig," is introduced and propelled through the pipe by the nitrogen pressure to clean the line. The nitrogen volume required to purge equipment with a simple cross section is determined using the following formula:
Where:
V Vo
V = VoP/14.7 = Total nitrogen vo volume re requir uired (s (scf) = Wa Wate terr volu volume me of of pip pipel elin inee (cf) (cf)
P
=
Absolute pressure of nitrogen in the pipeline during purging (psia)
2. Pressurization Purging
This method is used when conditions do not permit a sweeping action of nitrogen through the vessel. The vessel is repeatedly pressurized and mixed with nitrogen gas and then the mixture is exhausted. The total volume of nitrogen depends on the number of pressurizing purges required to reduce the contaminant to an acceptable level and can be determined by using this formula:
Where:
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V Vo
V = 1.2nVoP/Pa = Total nitrogen vo volume re requir uired (s (scf) = Water Water volume volume of vessel vessel or tank tank (cf) (cf)
P
=
Pa n Co C
= = = =
Absolute pressure after pressurization with nitrogen (psia) Absolu Absolute te pres pressur suree after after exhaust exhaust (psia) (psia) Number of purges = C log Co/(log Pa log P) Initia Initiall cont content ent of gas to be remove removed d Final content of gas to be removed
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3. Dilution Purging
This method is used for equipment cross sections such as distillation columns, kilns, reactors, etc. Nitrogen partially mixes with the gas to be purged out, and then the mixture exits through an outlet located as far as possible from the inlet. The nitrogen required to reduce a contaminate to a desired level can be found using the graph below and the total volume of nitrogen needed can be computed from the following formula:
Where:
V Vo n
= = =
V = nVo Total nitrogen volume required (scf) Water volume of equipment (cf)
Number of nitrogen volumes required
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PURGING EFFICIENCY (C/Co) AS A FUNCTION OF NITROGEN VOLUME (TANK VOLUMES) FOR VARIOUS PURGING TECHNIQUES
Purging Technique:
1. Displacement 2. Dilution Pf = Pi = 14.7 3. Dilution Pi = 14.7; Pf = 29.4 4. Pressure Purge Pi = 19.7; Pf = 24.7 5. Pressure Purge Pi = 19.7; Pf = 34.7
This information is for preliminary estimating only. Accurate estimations require a detailed set of site conditions. Simple cycle output is at ISO conditions with no HRSG and with DLN burner technology where available. Combined cycle kw output is using most favorable steram generation conditions for equipment. ISO conditions and inlet and outlet drops of 4.0 and 10 inches respectively. Some machines can use water or steam injection for NO x control or power augmentation. Such injection may substantially alter kw output. Base steam is at GTG base rate, open cycle, open cycle, with no supplemental firing for two points only for illustration. Most any combination of pressure and temperature is available. Remarks:
1. 50 Hz configuration available with no appreciable change in output. 2. Can be steam or water injected for additional power output and/or NO x control. 3. 50 Hertz only. 4. Burns heavy oil fuel only.
