First of all, I find the term 'imulähtö' somewhat ambiguous. The way I visualise this construction, 'imulähtö' is actually the inlet of a pipe through which a suction pump sucks a substance from the tank. In other words, the author here uses the word 'lähtö' in 'imulähtö' from the perspective of a substance leaving ('lähteä') the tank due to the action of the suction pump.

This exactly the opposite from the usual meaning of 'lähtö' in engineering, 'output'.

I am fairly sure that a substance enters into a suction pump via the 'imulähtö', be it a hose or a steel pipe. I would probably consider using 'suction inlet' as the translation for 'imulähtö'.

'Läpivienti' here refers to a penetration on the tank wall through which this hose or pipe goes.

So, when you have a pump mounted on top of a tank that sucks a substance from the tank by means of a hose or pipe, you need a flexible and waterproof penetration, such as a gasket element trough which the pipe enters the tank.

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Note added at 13 hrs (2018-07-25 08:19:39 GMT)
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Or, you could use 'penetration assembly'.

http://www.headhunterinc.com/product/tank-penetration-2/