Dumb PSV Question 2

[polyroly]

Good Afternoon,
I'm pretty sure I have the answer to my question already but I wanted to bounce it off this board to get some feedback. We have two PSV's in a closed loop refrigeration loop with calcium chloride as the fluid. We had an extended power outage and everything warmed up to the point that both relief valves popped. DCS historian data as well as my field visit verified that the safety valves opened.

My question. Do I need to take these valves out of service and get them recertified?

Thanks in advance,

polyroly

 

[The Obturator]

Not s dumb question. There is nothing obliging you to take the valves out. They done their job correctly. The only cause to take them out would be either a. If they are now leaking more than necessary (as a result of "dirty" seats), b. They did not open at the correct set pressure or c. Plant rules dictate it so. Essentially, there will be a point in time that these will require a service/maintenance of sorts, normally at plant shutdown. By correct examination at this point and noting all overpressure events prior, will you then be able to determine the true scope of future planned maintenance. This could be due to excessive leakage, corrosion, wear, ring setting etc., etc.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[georgeverghese]

Perry Chem Engg Handbook 7th edn says the presence of magnesium impurities in CaCl2 solutions can result in sludge formation in recirculating refrigeration brines - if this is applicable in your case, the sludge may impair further operation of your TSVs', if this sludge forms at the PSV inlet or orifice ?? If you believe this to be a significant concern, may be add a rupture disc upstream of the TSV, but this may entail resizing the TSV - shouldnt be a problem , given that the thermal relief rate is usually very small in comparison to the actual installed capacity. See API rules for guidance when using RD upstream of any PSV.

 

[polyroly]

Thanks for your response guys. I will refer to Perry's on the mg impurities George.

 

[georgeverghese]

Another approach would be to use a bellows type PSV for this TSV which the API recommends for fouling service.

 

[polyroly]

Hi George. Where in Perry's did you find the information about magnesium impurities?

 

[georgeverghese]

Got this from page 11-94, subsection on salt brines in the 7th edition. Here is a cut and paste from that subsection.

Brine of concentration x (water concentration is 1-x) will not
solidify at 0°C (freezing temperature for water, point A). When the
temperature drops to B, the first crystal of ice is formed. As the temperature
decreases to C, ice crystals continue to form and their mixture
with the brine solution forms the slush. At the point C there will
be part ice in the mixture l2/(l1 + l2), and liquid (brine) l1/(l1 + l2).
At point D there is mixture of m1 parts eutectic brine solution D1
[concentration m1/(m1 + m2)], and m2 parts of ice [concentration
m2/(m1 + m2)]. Cooling the mixture below D solidifies the entire solution
at the eutectic temperature. Eutectic temperature is the lowest
temperature that can be reached with no solidification.
It is obvious that further strengthening of brine has no effect, and
can cause a different reaction—salt sometimes freezes out in the
installations where concentration is too high.
Sodium chloride, an ordinary salt (NaCl), is the least expensive per
volume of any brine available. It can be used in contact with food and
in open systems because of its low toxicity. Heat transfer coefficients
are relatively high. However, its drawbacks are it has a relatively high
freezing point and is highly corrosive (requires inhibitors thus must be
checked on a regular schedule).
Calcium chloride (CaCl2) is similar to NaCl. It is the second lowestcost
brine, with a somewhat lower freezing point (used for temperatures
as low as -37°C). Highly corrosive and not appropriate for direct
contact with food. Heat transfer coefficients are rapidly reduced at
temperatures below -20°C. The presence of magnesium salts in
either sodium or calcium chloride is undesirable because they tend to
form sludge. Air and carbon dioxide are contaminants and excessive
aeration of the brine should be prevented by use of close systems.
Oxygen, required for corrosion, normally comes from the atmosphere
and dissolves in the brine solution. Dilute brines dissolve oxygen more
readily and are generally more corrosive than concentrated brines. It
is believed that even a closed brine system will not prevent the infiltration
of oxygen.