1 reply to this topic

[Milutin]

Dear all,

We have old HDS unit designed to hydrotreat light cycle oil from FCC unit. During the years purpose of unit is changed to light gas oil hydrotreating.
Low catalyst volume and low H2 partial pressure in reactor and strict diesel sulfur specification caused throughput reduction from originally designed 60m3/h for LCO to 20m3/h for LGO.
Recycle hydrogen gas flow stayed the same as during LCO operation, this cause unnecessary high H₂/oil ratio, high feed vaporization in reactor and unnecessary fuel consumption on feed fired heater which is also near maximum duty because less feed preheating, reason is less heat realized in reactor when processing LGO compared with LCO processing.

According to unit manual, quote: "Compressor is two cylinder reciprocating machine with inlet and outlet pulsation dampers on each cylinder. Compressor is provided with 0%, 25%, 50%, 75% and 100% stepped suction valve unloaders to save power when operating at turndown conditions."

Our intention was to unload compressor to 75% or 50% and solve above mentioned problems, but our consultant advises as not to unload compressor because that can cause vibrations and machine damage.
I am confused about this because unloaders are common devices for regulating reciprocating compressors flow, even in unit manual is written about possibility for compressor unloading.

Does anybody have idea how reciprocating compressor unloading can cause vibrations?

Regards,

Milutin

[Art Montemayor]

Milutin:

Vibration in a reciprocating compressor is serious business. Vibration caused by valve unloaders in a designed and manufactured compressor is an even more serious business if the machine was designed and assembled with the intent of using it unloaded. I frankly don’t believe what your “consultant” is saying because I don’t know enough about the application. You haven’t given specific basic data to define and explain what is occurring (or what is expected to occur). For example:

1) What are the make, type, and model of the compressor? What is the action of the pistons in both stages? What is the connected horsepower? How old is the machine?

2) Have you contacted the manufacturer and queried them about the predicted vibration? What is their reply?

3) Is the machine skid-mounted or installed in a concrete monolith foundation?

4) What are the make, type, and model of the valve unloaders? How are they activated – manually, pneumatically, or electrically? Is another capacity control system used in conjunction with the valve unloaders – such as speed control or discharge recycle?

5) Did the compressor manufacturer install and supply the valve unloaders? Did the machine operate initially with capacity control using the valve unloaders? Did the machine undergo an initial performance test under the valve unloaders?

6) Have you, personally, been at the machine and tried to make it work using the various valve unloader steps? Have you seen it vibrate yourself? Has anybody seen it work like the consultant says it will vibrate?

7) You don’t state it, but I presume you have a second stage intercooler and suction scrubber installed in between the two stages, complete with liquid separation and drain.

8) There are various ways to unload a reciprocating compressor at each stage and overall. One method is to use valve unloaders – and there are basically two methods of valve unloading:

• Suction valve finger unloaders; and
• Suction plug valves.

Which type of valve unloaders does your compressor have installed?

This is a very important issue and subject to resolve as soon and as safely as you can – if indeed, there is vibration caused by the unloaders. You should be able to safely and easily reduce the capacity and the resulting energy consumption of your compressor at will. I will await your reply.