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Kasim Flowers
Kasim Flowers

Blow Gas

The pipe to be cleaned is connected at one end to a gas reservoir. This can be a tank or upstream section of clean empty piping. At the other end, a temporary exhaust pipe and valve is installed. The reservoir is pressurized to less than half the design pressure. When the blow valve is rapidly opened, the pressurized gas violently rushes through the piping and out the exhaust pipe (Figure 1).

blow gas

As the gas moves through the pipe, debris and liquids are picked up and blown out. When the reservoir depressurizes to a level where no more cleaning is effectively taking place, the valve is closed. The process is then repeated over and over (10 to as many as 100 times) until the pipe is judged clean.

The majority of cleaning takes place during the initial round of gas blows. As cleaning progresses, the color of the discharge plume will change from rust orange to clear. Only after significant cleaning has taken place, should the target be inserted.

Manufacturers will identify how the gas inlet piping to their equipment should be cleaned. Gas blows are primarily performed on the fuel gas supply piping from the outlet of the gas service meter to the isolation valve inlets at the CT. If stainless steel piping is used downstream of the fuel gas conditioning skid, it can instead be visually inspected using a borescope. If debris is found, the stainless pipe should be blast or brush cleaned, as conditions require.

The length of piping that can be cleaned during a gas blow is limited. Flow resistance, and the location of isolation valves and branch connections, will usually require multiple flow paths. Each path will need time and resources to prepare and set up. Identifying the optimum blow paths is critical to having an efficient process and, therefore, requires thoughtful consideration to minimize the number of blows needed.

The diagram should also identify any equipment skids, flow nozzles, orifice plates, and thermal wells that should be removed, replaced with spool pieces, or bypassed to prevent them from being damaged or to eliminate the pressure drop they would cause during gas blows. If a valve cannot be removed, it should be fully open during the blows and occasionally cycled closed and open between blows to shake lose any particulates that may have gotten caught in the valve. The following is some advice for defining blow paths:

The blow valve is critical to successful cleaning. When closed, the blow valve is used as a stop valve to allow the upstream piping and reservoir to be pressurized. When opened, it initiates the gas blow cleaning process. Most importantly, the valve must be able to open quickly. A faster valve will allow more cleaning gas to scour the inside of the pipe at maximum velocity. A quarter-turn ball valve or knife gate valve that opens in 0.5 to 1 second (or better) is a good choice. An automatic operator is preferred for safety reasons.

The closer the valve is to the tail pipe; the more volume will be available upstream for pressurization. More volume gives greater stored energy and longer blows. Alternatively, locating the valve upstream of the piping being cleaned will provide an additional cleaning benefit when the valve pops open and the pressurized gas impacts the dirty pipe. However, most importantly, the valve should be installed away from flow disruptions, and if a manual valve is used, it should be in an accessible and safe location.

Rupture disks are sometimes used instead of a blow valve. They are disposable and open very quickly. However, because they must be replaced after each blow and are manufactured to a single predefined burst pressure, they may not be suitable when many gas blows or starting pressures are needed.

where mB is the mass flow rate of the blow gas during cleaning, vB is the specific volume of the blow gas during cleaning, gc is the gravitational constant, and A is the area at the tail pipe exit. (All variables in equations referenced in this article use customary U.S. units, such as ft, sec., lb, psia, and F, unless otherwise stated.) CF during the gas blow must be greater than the maximum CF during operation for effective cleaning to take place. In other words, the ratio of cleaning to operating CF, known as the cleaning force ratio (CFR), must be greater than 1.0, as calculated in the following equation:

where CFB is the cleaning force of the blow and CFM is the maximum cleaning force during operation. Assuring CFRs are greater than 1.0 is the fundamental criteria upon which the gas blow starting pressures are based. Experience has shown that drag forces from a CFR greater than or equal to 1.2 will provide acceptable results. Higher CFRs do not show significantly faster cleaning and are not recommended because more gas is exhausted. However, a higher CFR margin should be used if required by the equipment manufacturer.

To calculate CFR, the mass flow and specific volume is needed along the entire blow path. Sonic and subsonic velocities, choked flow, and density changes occur, which make the calculations difficult. Fortunately, compressible fluid flow software greatly simplifies the effort.

The objective is to use the model to find the blow gas reservoir starting and ending pressures that will result in acceptable CFR values throughout the length of pipe being cleaned. This is done by a trial and error process in which different pressures are tested, and the mass flow and specific volume results are imported into a spreadsheet that calculates the CFRs. The following is some advice for determining starting pressures:

where dE is the tail pipe gas exit inner diameter and r is the distance from pipe exit. If the plant is near a residential area, a silencer will be necessary. When specifying a silencer, pressure loss at the maximum flow rate should be restricted to no more than about 1 psid. A high flow resistance will result in higher blow pressures and more blow gas being used.

where P1 is the gas reservoir starting pressure, SG is the specific gravity relative to air (1.0 for air and approximately 1.0 for nitrogen), V is the storage volume under pressure (including tank, reservoir, and piping), and FC is the choke factor (1 for an ideal exit to 1.8 and higher for a typical gate valve). Ensure the blowdown time is sufficiently greater than the blow valve opening time and the reservoir is not pressurized above the rated pressure capability of the components.

where P2 is the gas reservoir ending pressure, Pa is atmospheric pressure (14.7 psia), and Q is compressor output capacity for filling the reservoir. To estimate the calendar days needed for cleaning, the number of blows required for each blow path is needed.

Experiencing a shortage of nitrogen during the cleaning process will delay completion. To estimate the quantity of nitrogen required for a gas blow (q GAS), use the following expressions based on the perfect gas laws:

Prior to performing gas blows, the piping must be pressure (leak) tested. Normally, water is used. However, if permitted by the piping code, pneumatic testing could instead be performed using the cleaning gas. This may be desired in situations where the hydro test water cannot be easily removed, cannot be tolerated in the pipe, or is not sufficiently available.

where mM is the mass flow rate of gas during maximum operation and MWT is the gas molecular weight (about 28.5 for air and nitrogen). When the blow valve suddenly opens, the pipe will be subjected to dynamic loads as the gas starts flowing. The dynamic forces are accounted for by a dynamic load factor (DLF). This factor represents the ratio of the peak stress from a rapidly applied load to the stress that would occur, if applied slowly. DLF ranges from 1.1 to 2.0. Typically, a DLF of 2 is used. If a lower value is desired, see ASME B31.1 for details.

If your engine is experiencing blow-by symptoms, major fixes may be necessary. Prioritize working on the affected engine, as blow-by issues can increase the longer they continue. A good first step would be to clean or replace piston rings, then clean or resurface the engine block cylinder walls. Alternatively, you may want to replace the engine or generator set altogether.

The CSB concluded that the practice of using flammable gas to clean piping is inherently unsafe, and that alternative non-flammable methods, such as blowing with compressed air, are efficient and readily available.

Early blowtorches used liquid fuel, carried in a refillable reservoir attached to the lamp. This is distinct from modern gas-fueled torches burning fuel such as a butane torch or a propane torch. Their fuel reservoir is disposable or refillable by exchange. Liquid-fueled torches are pressurized by a piston hand pump, while gas torches are self-pressurized by the fuel evaporation. The term "blowtorch" is commonly misused as a name for any metalworking torch, but properly describes the pressurized liquid fuel torches that predate the common use of pressurized fuel gas cylinders.

Torches are available in a vast range of size and output power. The term "blowtorch" applies to the obsolescent style of smaller liquid fuel torches. Blowtorches are typically a single hand-held unit, with their draught supplied by a natural draught of air and the liquid fuel pressurized initially by hand plunger pump, then by regenerative heating once the torch is in operating state. The larger torches may have a heavy fuel reservoir placed on the ground, connected by a hose. This is common for butane- or propane-fuelled gas torches, but also applies to the older, large liquid paraffin (kerosene) torches such as the Wells light.

Many torches now use a hose-supplied gas feed, which is often mains gas. They may also have a forced-air supply, from either an air blower or an oxygen cylinder. Both of these larger and more powerful designs are less commonly described as blowtorches, while the term blowtorch is usually reserved for the smaller and less powerful self-contained torches. The archaic term "blowpipe" is sometimes still used in relation to oxy-acetylene welding torches. 041b061a72


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