Iam presently transfering sulphuric acid from Drying tower of 93% concn at 70'c and also acid from Absorption tower at 98%,11o'c using chrome cast iron pipelines at a velocity of 1.6m/sec.As i increase my plant through put i will have to transfer more acid .What will be the max velocity that i can go up to without causing any leaks.Can i go up to 3m/sec or should i restrict to less than 2m/sec by changing the pipelines?
chrome cast iron i use ,has typically the following composition:
C = 3.2 to 3.6%
Si = 1.6 to 2.4%
Mn = 0.3 to 0.6%
P = 0.2 (max)
S = 0.08 (max)
Cr = 0.5 to 1.0%

thanks and regards avadiappan

avadiapan,

There are two different issues in regard to the acid flow velocity in the iron piping. First, the corrosion rate of the chrome cast iron will increase as velocity goes above some threshold value. I am familiar with a ductile iron acid piping product where the smaller sizes might have a 1.6 m/sec velocity limit, and larger sizes (350mm+) might have a 3 m/sec velocity limit. The existing acid piping could have an extra 10%-15% margin for increased velocity before it would have concerns for accelerateed corrosion and leaks from loss of wall thickness. It would be good idea to verify how much wall thickness remains in the existing acid piping before pushing higher velocity in the system.

The second concern for higher flow velocities in the existing acid piping would be the increased friction and pressure drop from the higher velocity. The pump horse power could be increased by the square of the ratio of flow velocity [(v2)/(v1)]**2. or almost 3.5X, not just the percent increase of flow velocity. Do you really want to have to pay for that much larger motor and energy usage along with the larger pump? I would think the existing acid piping would best be replaced with larger sizes for the increased flows.

If the pump and motor are not a concern for the higher flow velocities, then you may want to consider some of the alloy piping systems for their much lower corrosion rates at higher design velocities. The piping sizes might even be smaller than the existing acid piping to result in a reasonable cost for a replacement alloy piping system.