5 replies to this topic

[Rahul Valimbe]

I am working on a tankage facility. I am having HSD. I want to transfer this from the storage tanks to tankers in bay area via dedicated pumps. The conventional way of doing this is by flow control valve at the discharge of the pump getting feedback from flowmeter installed in the line.

 

I want to explore the possibility of controlling this by the pressure in the tanker. I am aware that flow meter serves dual purpose of flow control as well as metering. But, If we go for pressure control, that would avoid level controllers and may give a precise control.

 

I would like to know the pros and cons of this method and has anyone gone for this before?

 

Thanks in advance.

 

Regards

 

Rahul

 

 

[ankur2061]

Rahul,

 

The way I have seen Tankers for white oil products being loaded is a flowmeter with either a control valve or a 2-step valve. Control valve is self-explanatory but many might not be knowing about 2-step valves. A 2-step valve is a kind of on-off valve. When the flowmeter is set for a desired quantity and the tanker filling is started the flowmeter starts counting backwards with the 2-step valve fully open. When about 5-10% of the set quantity remains to be filled, the 2-step valve closes 70 to 80% and reduces the filling rate of the final 5-10% to be loaded in the tanker. When the flowmeter reaches zero while counting backwards or in other words the desired quantity has been loaded, the 2-step valve shuts-off, stopping the loading. The 2-step valve thus helps in loading precise quantity of material in the tanker and also helps in preventing over-filling.

 

When you are filling a tanker by the bottom loading method (preferred loading method for all new installations), the liquid level is constantly rising with corresponding rise in the back-pressure due to static head at the inlet connection of the tanker. A pressure control would not work in such a situation because of the dynamic change in the back-pressure over the loading period.

 

Hope this helps.

 

Regards,

Ankur.

[Rahul Valimbe]

Thanks ankur for your valuable input.  

 

The method you suggested is apt for this application.

 

But, dynamic change in backpressure would still be there in case of flow control, as well.

 

Regards

 

Rahul

[ankur2061]

Rahul,

 

That is the reason the 2-step valve is a better way of loading trucks compared to a control valve. A control valve may hunt due to varying back-pressure while filling causing the filling rate to vary erratically.

 

Regards,

Ankur. 

[Rahul Valimbe]

Thanks for your help.

 

Regards

 

Rahul

[TS1979]

Hi Rahul,

 

It seems that you are building a facility to fill a tanker and you want to use pressure control instead of flowrate control. Let's asmue that your pressure control scheme works, which pressure do you try to control, pump discharge pressure or tanker pressure? If you want to lower the pump discharge pressure, the pressure may not be enough to keep certain flow rate to the tanker. If you increase the pump discharge pressure, the energy you put has to be consumed somewhere - means that the liquid will shot inside the tanker resulting in erosion problem.

 

If you want to control pressure inside the tanker, there are two operating modes, tanker with balanced line or pressurized tanker. Pressure inside the tanker with balanced line will be constant (or slightly different by the static liquid head, for 2 diameter tank, the pressure difference is about 3.5 psi) and you cannot control the pressure. For the closed pressure tank, the pressure inside the tank is determined by the liquid level, vapor space and vapor pressure. You can control the tank pressure through certain system such as using air, nitrogen, or even vacuum system but you cannot tell from the pressure to tell what the liquid level is inside the tank.