Calculation of upward force on a bed limiter ?

[jamesbanda]

I've gone through the kister book and noted that he mentions a stnadard upward force of 20 lb/ft (0.0096 bar) to 30 lb/ft (0.0144 bar) as the force typically designed for..

Kister also mentions that a pressure surge / change of vapour load increases the requirement for mechnaical strength..

so how do i calculate this..?

> Do i take expected DP then convert to a force..?

For a xx mm bed i've been quoted a weight for the hold down plate tollerance of 100 kgs (say for illustration).

So do I
Force = Mass * g => 100*9.80665 ~ 1000 Newtons
pressure=force / area
does area = free area of hold down plate for flow or area for non-flow.. to me it makes sence it is the non-free area for flow.. as that is what the forces acts on.. the blocked area..
free area = 0.14 m2 (say for a 0.6 ID Bed. with 50% free area.)

so max tollerable Dp =f/a = 1000/0.14 =6900 pa or 0.69 bar

Is this the correct method..

so for a pressure surge what method should be used ..? how do i calculate the lifts / anayslsis..


20 0.0096
30 0.0144

 

[BlackSmoke]

had a similar issue a while back where in an upflow gas-phase metal catalyst bed, the johnson screen was being damaged during startups. The initial velocity of the bed as the gas flow was turned on caused the bed to slam in the johnson screen roughly as a solid plug, causing the cantilevered supports to bend. Really the issue here is that you get nearly the gas flow's velocity with a drastically higher density of the metal. However, it's a bit of a difficult calc as it's dependent on the characteristics of the bed, i.e. the instantaneous force on an elastic ball hitting the floor is different than a inelastic ball.

We ended up solving this with a grating that broke up the bed "slug" and operating procedures which ramped up the feed, so we never got quantitative with this one.