High TAN Crudes 1

[SeanB]

As with most refiners today we are processing heavier crude slates, with much higher TAN. Our vacuum tower bottoms pumps have experienced significant corrosion - believed to be due to naphthenic acid attack. These pumps are to be replaced and it is requested to upgrade the metallurgy. The pumps are already constructed of 317L SS - which is generally considered to be resistant to nap acid. Our next option is to upgrade to Alloy 20. I was just wondering if any of you out there are experiencing similar issues, and what metallury are you using in this service. Thanks in advance.

 

[SJones]

API RP 571, sub-clause 5.1.1.7 and Table 5-2.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[SeanB]

Thank you for that reference. I now know that Alloy 20 would not be a good selection - it would actually be a downgrade as it has a lower Mo content than 317L.

 

[CJKruger]

SeanB,

1) Are you sure it is naphthenic acid corrosion (for the vac bottoms) ? I believe I saw some data that suggest the TAN for vac bot is lower than the gas oils, and the sulfur provide some protection.

2) Maybe it is temperature related. What is your vac bot temp ? If above 350 C, you should perhaps quench it.

 

[EmmanuelTop]

I agree with SeanB. Vacuum residue is usually not so much exposed to naphthenic acids attack as side-cuts from ADU and VDU (LGO, HGO, LVGO, HVGO). It can be high temperature sulfidic corrosion in your case.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[SeanB]

Our vac bottoms runs at approx 680 °F. According to API 571, Nap Acid Corrosion is typical in vac tower bottoms piping. Piping systems are particularly susceptible in areas of high velocity,turbulence or change of flow direction such as pump internals, valves, etc. In our case it is the pump that is experiencing the corrosion.

 

[0707]

Naphtenic acid is the generic name to define all organic acid contained in the Crude. The majority of these acid are known by the chemical formula R (CH2) nCOOH where R, is a ciclopentano ring and n, is typically bigger that 12. However, a multiplicity of other organic acid composites are also present in crude oil and until to date nor all had been analysed. Typical Naphtenic Crudes come from: California, Venezuela, North Sea, west Africa, India, China, Russia.
TEMPERATURE
Naphtenic corrosion occurs at temperatures between 200ºC and 400ºC.
CONDITIONS FOR the OCCURRENCE OF (NAC)
In general, the conditions that promote the occurrence of Naphtenic corrosion are: TAN, temperature, metallurgy of the equipment and velocity of the flow and presence of H2S.
TAN (Total Acid Number)
As general rule when the TAN of the feed is less than 0.5mg KOH/g Naphtenic corrosion will not be expected. If the rate of sulphur in crude is too much low or either less than 0,2% in weight, there is a strong probability of Naphtenic corrosion occurrence. In this last case, number TAN of crudes to process, between the feed and the cuts, will have to be placed between 0.3 and 1.0.
The methods of Mitigation of Naphtenic corrosion are; make blending of the crude mix in order to lower nº TAN of the feed, injection of phosphorous inhibitors and make "upgrade" of known resistant alloys “immune” to naphtenic corrosion such austenitic 317L.
Some are of the opinion that continuous operations Technologies of sulphur-based corrosion inhibitor, in monitoring naphthenic acid corrosion have the same value as phosphate ester programs and are not so prone for catalyst poisoning by iron compounds.

Luis Marques

 

[SeanB]

I just wanted to re-visit this thread. I am hoping someone out there may be having similar experiences. Our vacuum tower bottoms pumps are currently constructed of 317L SS. These pumps have only been in service 8 years and have had to be replaced. The pumps were inspected by our metallurgist and he has said that is the corrosion is a clear cut example of nap acid attack. We are processing crudes with TAN as high as 3.01. I would like to hear from anyone who may be experiencing similar problems.