This is one of those "how long is a piece of string?" questions to which there are no easy answers.
There are perhaps 10-12 major companies that specialise in providing these skids.
The main criteria is usually capital cost and it is a competitive market where margins often aren't that great which takes some of the flexibility away from you.
Many of the companies who build these skids can supply at very competitive prices because they build significant numbers of them.
They (hope to) make money on service and maintenance over the life of the skid).
This is as it has to be because buyers worry about capital costs when buying the things and not about operating costs. That comes later, once they have them on site and in use. In a repeat order business, sooner or later you may think you will have to deliver economies on both capital costs and operating cost, or a demonstrated balance between them but in reality, capital costs often dominate purchasing decisions.
You need to find just the right balance and it isn't simply about size but also about quality or suitability.
All too often clients are penny wise and pound foolish (or is it "Look after the pennies and the punds will look after themselves"?).
When building a new supertanker, costing millions, they will not hesitate to invest a significant amount of time trying to scam the supplier out of another 20% from a piece of kit that costs $5-6,000 and then pay 50% more in operating costs.
For HFO skids the capital costs dictate the choice of flow meters used. These often prove troublesome and expensive to operate but the capital cost of a better flowmeter, one that would probably deliver minimal in service costs and superior on-stream factors is a tough sell.
Much as you might want to specify a rotor type meter the client may only want to pay for a sliding vane meter.
The heavy fuel oil skids will also have viscometers for fuel heater control. These are a one size fits all but they pay OEM prices for components which can be as advantageous as 1/3 the unit prices.
This is fine with a one size fits all item but when you get to the flow sensitive items, you can't simply standardise on one size because what happens is that the smaller skids for which it is over sized become uncompetitive.
But where these skids have a lot of components that are very sensitive to flow capacity such as the flow meters, heat exchangers, pumps, control valves etc., standardising is a tricky problem.
Some of these are low cost items and can be sized for the application because the numbers game doesn't deliver enough advantage from standardising. Others you may have to adopt a degree of standardisation simply to make the numbers game work for you.
So, for a given market share or number of systems supplied, standardising makes sense when the compromise is that you cover the eight different flow ranges you mention with, say, 3 or 4 different skids, if by so doing you can win on the supplier discounts on bulk orders for common components and can sell the end product.
If you supply enough of these skids then that 3 or 4 skids can become 8 different standard skids because each skid is more cost competitive for its specific application than a "standardised" skid that can serve two applications - once you get the numbers right.
And that is part of the problem, it is a numbers game.
But in any event, this is often a thankless task because you have to find a path between two competing ideologies... manufacturing and purchasing who want to produce a "one size fits all" solution so they can keep inventory down, minimise drawings and documentation preparation, and where purchasing can pressure suppliers for better prices (often in return for purchase contracts that agree to take a minimum number of units per year and take them in reasonable size batches at regular intervals).
Oh, and of course the clients, each of whom wants something special.
A good fit is where you are able to homogenise the hundreds of idealised individual schemes into a minimal set of standard solutions where the client will compromise his needs for the price advantage he gets by accepting a standard package.
How good a fit you achieve is only revealed when he buys from you and not your competitor.
So the question you ask is one that depends on a very wide range of factors, not least how effective your purchasing department is, how good your sales team is, how many you make, how flexible your client base is and how good the competition is.
The problem for the design engineer is to try and bring all these factors together in a reasonable way and satisfy his various "customers" and the design engineer's customers are not just the client who buys the skid but internal customers such as purchasing and manufacturing.
You will have to allow others to share decisions about which manufacturer to use for a meter or pump because they can get a better deal with one manufacturer and yes, they may be trade offs in operating costs, reliability and so on but that is the reality. It is more of a reality in one market segment than in another.
It isn't just an engineering solution but one which depends on how good a team there is working with you in each of their different skills.
This may mean, should mean, that you need to run up a few different options and then discuss with the other "clients". You need the input from the different players on your team.
This will be a balancing act where what works for you and your team will be a quite different solution to that which will work for one of your competitors.
That isn't necessarily a bad thing, it means that while you may lose the sale to one client because your solution is not so good as the competitors, you will win with another client where you and your competitors offer the same things but your solutions are seen as a better fit with their expectations.
What is sad is when your solution is rejected by all the clients or by too many of them, upsetting your projected sales or the sales split between sizes and thus messing up all those carefully worked out purchasing deals.
JMW