Trapped Water Risk 2

The most hazardous locations where accumulated condensate can create problems when unit is put back on line, are:

- Reflux and pumparound pumps
- Pumparound heat exchangers
- Flow control valves (stripping steam, pumparound FCVs and TCVs, overhead reflux etc.)
- Provide for self draining of accumulator trays (especially welded chimney trays)
- Lowest pipeline locations (and dead spots) in the unit
- Check for all possible locations where steam can condense and make pockets of condensate, that can be introduced later on the hot hydrocarbons

We have seen a particular problem with water accumulated in the bottom of pump cases. Primary concerns would be coker charge and FCC charge. The problem occurs when a pump is going to be placed back in service. On these hot pumps we usually have drilled check valves to keep the pumps hot. So, when they open the block valves on the standby pump, hot product comes through the drilled check. When it hits the water, steam is produced. The steam bubble then passes up the suction line, crosses over through the interconnected piping and hits the running pump. This causes the running pump to gas up and loose flow and the unit trips off on low flow. The water can accumulate from the normal process of steaming out a pump to get it ready for maintenance. If the pump does not have a drilled check valve, then the water may not flash until the pump is started up. Then the steam is pushed into the discharge. As the steam bubble passes through the flow meter, it will result in a false indication of low flow which can cause a process upset. If the water hits the hot fractionating tower before it flashes, the resultant steam explosion can damage trays and tower internals.

Johnny Pellin

In addition to others advice you have to impeach as much as possible water and hot condensate entrance into both crude and vacuum column during start-ups. So before oil in you should carefully steam purging all unit at low points to vaporize as much water as possible to avoid water blow ups in contact with the hot oil. Also during feed unit tanks changing, process and storage operators should work close together to correctly purge the feed tanks and supervise dessalters operation minimizing suddenly undesired water pockets, which if they reach the columns will damage packing, trays and other columns hardware obliging unexpected shut downs for maintenance repairing.

Luis marques

Ok.Are there any case studies available based on above risks ?

My answer is yes! you just have to look for them.

I have searched NPRA Documents of last 6-10 years but I could not find any case studies based on that.Can you pl. throw some light on these ?

Have a look at Trevor Kletz's HAZOP and HAZAN - Identifying and Assessing Process Industry Hazards. If I'm not mistaken, you may find he has reviewed some cases.

"17. Q: Is special attention needed during start-up?
A: In order to avoid damage to equipment, several of the common causes of problems will be noted here. These are well known but frequently overlooked:

High liquid level in the column
If the liquid level is too high, vapor velocity causes massive entrainment to the bottom tray. This entrainment may even be in the form of waves of liquid. This constant buffeting of the tray loosens bolting and can dislodge the tray.

Too rapid liquid drainage
When a column floods, the liquid level may rise to a point several feet above the bottom tray. If the liquid is withdrawn from the bottom of the column at a rate greater than it can flow down through the trays, a vapor gap will be formed below the bottom tray. The presence of a vapor gap below the bottom tray, however small, imposes on that tray the weight of all liquid above it and could cause immediate failure. This is fairly common during start-up when the column becomes flooded and the immediate reaction is to lower the level as rapidly as possible.

Steaming out a column
Many columns are operated on steam-water prior to start-up in order to check out instrumentation. Caution must be exercised to assure that steam is not condensed during this operation. Condensing steam can result in a downward-acting differential pressure across a tray which may exceed the mechanical strength of the tray and cause failure.

Water in a column
Many trays are installed and tested for tightness and may even be seal-welded to insure against leakage. As a result of the tray leakage tests or from washing out the column during the shutdown procedure, water may remain in the column. If the feed to the column is extremely hot, this water left in the column may be vaporized instantaneously, causing an increase in vapor rate. Liquid may also be lifted into the trays above. The net effect is damage to the trays.

Pressure surges
All columns should be designed so any change in pressure causes an upward flow of vapor instead of a downward flow. In the case of vacuum columns, the valve used to change from vacuum to atmospheric pressure should ideally be located near the bottom of the column. If the temperature or contents in that zone prohibit location of the valve in that area, the valve may be located at the top of the column. However, its size must be restricted to ½ - ¾". A larger valve may permit too large a quantity of air or inerts to enter the column. If trays have a liquid level on them, the vapor will not be able to flow downward through the tray freely. This can create a high pressure differential and cause failure. Similar situations have been noted where rupture disks have been located in the lower part of the column at pressures above atmospheric."

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luismarques

Thanks Mr.Luis & Thanks to Every one !!!

Due to many of the risks listed above sometimes it can be advantageous to O2 free a unit on startup with nitrogen, if a large enough supply is available, instead of steam. This helps to buy down the risk of pulling a vacuum due to condensing steam in an isolated section and rapid, potentially damaging, vaporization upon hydrocarbon introduction. However, if nitrogen is to be used for O2 free and leak check activities, the proper safeguards must be in place to mitigate the hazards associated with potential oxygen deficient environment in the unit, such as O2 badges, gas analyzers, etc.